Plex hasn’t said this publicly, but it is my speculation that the change is due to legal reasons.
Over that past year, Plex has been receiving a lot of DMCA takedown requests for posters (you can search the Forums for these) causing posters to go completely missing/blank. My guess is Plex made the decision to properly license posters from Gracenote instead of using TMDB so they can solve this legal issue. Afterall, it is better to display some poster instead of having a blank poster, even if that poster is not the “best” one.
If I am correct, then this change is here to stay and we all just have to live with it. We can still change the posters ourselves. Plex hasn’t taken away that ability and there are plenty of other options to choose from. You can still select a poster from TMDB and you can automate it with my script posted above.
