Marketing is marketing. Trying to sell a product that's yet to be finished can lead to this situations, especially when combined with publisher's need to hype everything up. Considering how big preorders are and how fast the value of digital content decreases, there's no real reason for publishers to stop this. Hell, sometimes it's inevitable because the game is not finished and they are trying to sell you a product way before completion, so they have to show you something. With that said, they are still at fault for trying to sell the pelt before hunting the bear. Best advice is always to wait, there are thousands of games out there, no need to preorder anything anymore (unless they are limited retail releases, and even then those are niche titles that rarely promise the world with their marketing). Trailers can promise you the world, but if you wait a month to buy the game, you'll see if they are telling the truth.

I'm going to disagree but only because this wasn't the case with Hellblade 1 when it was first revealed vs released.

If anything the graphics were vastly superior in the final release build.

This in 2014

vs This in 2017

The problem is there is no games made using Xbox One X or PS4 pro as baseline. All games coming to PS4 pro and Xbox One X are running PS4 and Xbox One original games (using base console as baseline).

So you have seen nothing. 

Just look this Unity demo made using  GTX 9800 hardware which is equal to PS4 pro and less powerful than Xbox One X, this demo look far better than any games coming to 8th gen consoles. Because this demo  are made using GTX 9800 as baseline.

Imagine if games are made using  Xbox Seires X as baseline , well you don't have to imagine , Hell Blade is made using Xbox Series X as baseline. 

You have to remember what the terms mean when they are shown off. In-engine is actually running on the HW, or supposed to be, but it's using extra effects the gameplay sections won't have or will have pared down. The system is rendering predetermined events, as in cutscenes, and has the extra power to do so. Personally, I can't wait to see what the first party devs at Sony pull off on the PS5.

lol when did that ever happen?
True next gen games always look noticeably better than their predecessors on last gen systems. Not just "a little bit".

Me either , I cannot wait to see another KillZone Shadow Fall moment again like in 2013, when people doubt the games run on real time LOL. 

I don't think about it much because I am more hyped about the games and the potential of a significant specs boost in general.

Will they achieve the exact visuals of the trailers? Maybe some will and some won't. Either way, it should be a significant upgrade.

We never see the best of the hardware at launch either. Eventually they may create games that look even better than those trailers.

Ninja Theory is pretty square when it comes to what they say they will do. They're amazing.

But keep in mind that there's also times where, over time, what is initially shown is nowhere near the potential the hardware reaches further down the line. That's a problem the Xbox 360 faced. Everything they showed looked like a minor leap and everything the PS3 showed looked so much more advanced. For the first year, people just called the Xbox 360 another Dreamcast (half step towards real next gen).

There's plenty of trailers that are misleading, even after the hardware launches. We just have to be vigilant. Don't believe everything you see but don't doubt everything, either.

I don't think the intentions of these developers is to fool us, it's just that often times things happen. By that I mean hardware plans change, devkits often times don't fully represent what the actual release hardware will be and based off of those changes, decisions have to be made. Heck, even changes in how the software is programmed can change the course of the visuals of a game.

That being said regarding Hell Blade I think the jump in GPU and CPU warrants what Ninja Theory displayed at the game awards. Game artists are getting better and better as more software techniques and new software in general pop up. It's just polygons, maps, and moving some vertices around, it's not that unbelievable. The lighting also does more for these games than most think. I also think Ninja Theory has been working on this for quite some time so I think what we saw is essentially what we'll get.

Once we see what Sony has to show off I think we'll be able to start piecing together the plausibility of how early 9th gen games will look. So, while I do think there will be games that look different than their alpha or beta footage, I do think it's to be expected and not necessarily deceptive marketing. That type of marketing leads you on until you buy it.

Nintendo eventually showed off Breath of the Wild in its true form and let people play it before release. If the game got no pre-release footage and was just Nintendo putting out renders of gameplay that wouldn't be possible on their base hardware then I'd be concerned about their integrity but that obviously wasn't the case.