Well I don't 100% agree with your FFXV statement. The E3 2016 gameplay demo (not the Titan one) was a mess and it got a great visual update after the delay and when it was released. If you're instead talking about the E3 2013 trailer, than it's another matter.
Note, looking worse does not=the final game looked bad. As it was still hands down one of the best looking JRPGs released last gen. It's just that they overshot a bit when it came to getting scenes trailer ready.
The trailer from TGS 2014 still looks insane, and not just because of the better skin shading, scene density, post processing quality, etc. but just in general the proposed scale. Like being able to ride past fields filled to the brim with wildlife and seemingly being able to get out at any time to attack them.
View: https://www.youtube.com/watch?v=ok4sTLzbLcs
Looking back it's comical just how many animals are in that car shot.
And ofc i'm not saying that everything looked worse, animation blending quality in the final game is better as an example.
I didn't bring up the 2013 trailer specifically because a metric fuckton of it is CGI target renders of the Altissia set piece with a fake HUD overlay mixed with a prototype build of a small battle sandbox that was indicative of how close FFV13 was to being cancelled.
