I haven't played Zestiria yet, but I loved Xillia 1 & 2 and felt invested in the character and story and loved battling in them. To me Xillia 2 question of murdering millions over and over to save reality was interesting to me.

Im not going to lie; I fell out of love with the series.

Phantasia - Awesome
Legendia - Awesome
Destiny - Awesome
Eternia - Favorite

Symphonia - Generic
Symphonia 2- Blah
Abyss - Blah: Jade Kurtis was awesome though
Graces - ZzzzZzzZZz
Radiant Mythology - Decent.

Tales needs to do something radical. It feels like a "How to make Tales game template" is just lying around, and they just fill the blanks with new, generic and soooo generic you can guess personality by just look at the design, character designs. Shake it up

So, i do agree that the series is going downhill, but i have a different view on the why.

Immediatly striking me as odd is that you didnt seem to enjoy TaotA much when your favorite is Symphonia. They are very similar games. Maybe it was because of the small 3DS screen?
I have not played Hearts yet ( i havent had time but i have it and tried it out a bit) but i'm not suprised it turned out to be good.
And finally you say the combat of Tales of Graces F is good, and i see some people saying this, but i completely disagree and i think it was the beggining of the downfall.

So, immediatly i will say what the reasons are. They are the combat, wich with every new games gets farther and farther away of what made it unique and the removal of the world map, wich just increases boredom time.

So, combat first. The classic 2D gameplay like system, may seem dated and simplistic, but in fact it isnt. It's fast nature with precisesiveness and the management of your TP and MP is what makes it singular. The choice of how you are going to combo the most out of your opponent and wich artes to use with each other to try and extend the combos, while minding your health and resources.
Now, on Graces this all went out the window. You had no TP. The character basically spams a string of attacks and all you do is sidestep or weave an arte in there. The control level is way lower than it used to be because the combat accelarated to the point where you lose most control of your character actions. Not only that, but all these artes didnt feel rewarding because in that game you needed to attack at the same time as your party member to see a damage reward. So, you are no longer simply in control and managing. You are hitting repetitive wet noodles that do nothing by themselves until they are coinciding with a party member. You have little to no control and it doesnt feel rewarding. Does it look flashy? Yeah sure it does, but its very frustrating.

The other thing that disappeared was the world map. This is an effort to follow FFX and that series as a whole.
I have never understood this move. It was always a poor one for either series. A world map gives the world a sense of scale that a connected landmass can't. It makes the game bigger, more epic and allows you more development time to focus on the fun things the player will do, rather then the how they are going to get there.
But Graces still managed it better than later games, but it obviously gave it the feeling of a very small world.

Queue in Xillia. They do the right thing and go back to 2D gameplay. The game garners alot of hype and launches to save the series. But, a nasty suprise awaited us. The field in between zones are now vast and filled with nothing but collectables. The story still progresses slowly because of this. The battles became easier than ever and end with you doing just a combo or two unless you are fighting a boss.
Xillia saved the series with the return to form in the combat but it was unbalanced, unrewarding and there were new flaws that are difficult to overlook. The extra development time for making the bland large areas had consequences on the development of the game's story and characters. The realistic models had difficulty portraying the anime style the series is know for and looked like a step down.

Now, the problem here is that Xillia gave them the wrong idea. And not only that but fans saying the Graces combat is good also gave them the wrong feedback.
Comes Xillia 2 and the same problems remain, plus they add a mute character and a frustrating game progression system in the debt system besides re-use of the same areas. Sales suffered.

From that the tales team concludes: It must be the combat!

Come Zesteria, lets go back to the graces system but with all the same problems still intact. Big boring fields of Nothing. Spammy uncontrolable combat.
It was the worst of both worlds Xillia and Graces put together.

Now we see Berseria and we see that nothing major is set to change. The series has been declining in sales and is aproaching dangerous levels once more. With their current sales they will never have enough budget to build a proper open world if thats what they want to try and do and they are too clueless to understand that they are trying to change something that worked.

....... The problem with Tales is that it caters to only a small fanbase. You cant expect major changes when you the shut the door to criticismon any non Tales diehard. Star Ocean is better with advancing because they develop games with a global mind and will global criticism.

Tales has a fanbase of 1 million gamers in Japan. Its not that the base is small. It's that they are failing to capture it. It's exactly the opposite. Its because they changed the game to a state where its hadly recognisable by fans that its falling from grace. You say that the solution is to westernise or appeal to those that don't enjoy JRPG's? How does that work?

Poor star ocean. I enjoyed SO4 for alot but it was slammed in the west. I don't think SO5 will be any different.

Im not saying westernize it? Tales leader has made it quite clear that he only values the opinions of the diehard Japanese Tales fans and aims the series at them, but there are other players as well who may not be as complacent as the diehards and when you shut the door on all those criticism because they're not your diehards fans your shutting out room grow. 

  In regards to Star Ocean, it'll definitely do better than any Tales game. 

I don't share your opinion on SO5. I think its gonna bomb. Not that it can't be a good game, but because it won't appeal to everyone.

I do think the visual presentation (cell shaded) does anime JRPG's a big favor, and its something the dev's in many cases seem to ignore, but thats another debate.

So, i don't think the Tales team is actually listening to fans very well. If what we have now is what hardcore fans want, then these hardcore fans aren't the same ones that enjoyed the eras os Phantasia and Symphonia. Because what we have atm, is quite honestly, mediocre. If its the result of harcore fan feedback, then sure. they need to listen to other people.

The solution is clear as day though. They don't do it cause its not what they feel like doing i suppose. Its their choice to gamble with their jobs.

There is a serious lack of love for Vesperia in this thread.. Is it because it's only in English on the 360?

Have you given Tales of Xillia 2 a shot? It's not my favorite in the series, but they threw the gameplay and system formula completely out of the window and really tried a lot of new things with it. I recommend you try it just to see the (new for Tales) decisions they made. I didn't like all of them (silent protag, segmented chapters/debt ceiling), but I liked that they ventured out a bit.

Yes, Tales has turned to crap. I agree with everything except what you said about Abyss, which is top 3 in the series and better than Symphonia to me. The original is still the best though, because it was more original. Every Tales game after just tried to follow, but never lived up outside of the battle system until Xilia and the awful Zestiria. I have not played Zestiria in a few months once I got Fallout 4, because of how bad it is and I think FO4 is close to mediocre, but much better than Zestiria.