As long as it's just a small fraction of the game, or a one time thing. A lot of games have little side missions which are different from the main game (ie - Chocobo racing, snowboarding from FFVII).

I have no issues if it's minor, but I really wouldn't want to see 5-10 stealth missions, personally.

Yes, the very gameplay the game is based on so it's no longer a turn based RPG. Improve gameplay, sure, don't just change the very gameplay the game series is based on else you might have well made it a spin-off..... oh wait....

FFXII had a more MMO RPG style of gameplay, far from action at all. The was ATB still present, you just wandered around open areas and big battles were still time turn based. Even then I didn't like this in terms of FF traditional gameplay. The game was fantastic and 15 might also be fantastic but that doesn't mean people don't want what they originally liked about the series, that being the gameplay establised in 2-9. 

In my opinion, it seems SquEnix are doing all they can to make FF less like traditional FF as they can and they don't seem to understand why people complain each iteration. I'm sure there are people that would love a action based RPG FF game, much like there are those that love FF11 and 14 as MMOs but there is a reason why I haven't played them.

I get what you're saying, but I really think changes are necessary and people shouldn't complain a lot about it (unless they think the change is for worse). If they kept the same turn-based combat forever it would probably hurt the franchise (in terms of sales and relevance). Plus, if some fans really want the same thing all over again... there are already almost 10 FF games that do that... just replay them. 

So Final Fantasy + Tales of + Thief + MGS + Gears of War + Halo = Final Fantasy XV? I am even more sold!

Let XV have some addition like that, then watch some people losing their minds... Final Fantasy has lost its way
It is an addition to change the pace of the gameplay. From the looks of things, it was completely optional. The player could just engage the enemies. More options is never bad. Also, Stealth can be useful in many situations, like if the player is low on supply or under-leveled.

+ GTA

They aren't necessary though, FF gameplay just needed to evolve, not be replaced. Long standing game series have had success based on the traditional gameplay that made them popular. Compare Street Fighter now to 20 years ago, sure, it's prettier, there are more 'supers' but in essence it's still a 2d fighter, a hadouken is still quarter cirle + punch. It's evolved and for the better.

I guess as a fan, I'm not allowed to have a game that plays like 4-9 with graphics of a modern game because 4-9 exist already? I guess I can just continue playing Street Fighter 2 too right?

Street Fighter is on its 5th main game tho, Final Fantasy is on its 15th. Btw, the fighting genre is much more limited than a RPG, maybe that's why they are not so hot anymore; I mean, your example, SFV, needed help from Sony to get funded.

What VXIII said.

 FF games all have their core battle system, but they have been known to have  a huge variety of sectional gameplay specific for only one part of the game.  FFX might be the only exception off the top of my head with the cloister of trials puzzles happening in every temple to acquire Aeons.

I do wish FF games did more consistent well thought out puzzle sections like the CoT from FFX though.  They have their mini games all over the place, but there needs to be more alternate and yet consistent gameplay sections central to the story.  Puzzles compliment RPGs well.