Ok., the dataminers have new info
THERE WAS A SIX MONTH LATER EPILOGUE, instead of 1-3 random lines from random companions about maybe jokes about what just happened or maybe them saying what they'll do and a nice scene for maybe 1-2 characters, there was supposed to be an EPILOGUE with catching up with your party 6th months later and finding out what they've been up to since!
Hopefully it is a bug not a choice to rmeove this and once they fix it it'll be back as seriously, having no ending was bad enough but having no ending AND THEN there being a good ending in the files just sitting there unused??? hells below
I think there is good reason to assume the situation will be fixed in the (near) future. If you look in the game files (an extract of the decompiled dialogue files can be found here) you can find;
END_GameFinale_CompanionFates
Withers has gathered the companions together one final time as a reunion of sorts 6 months later. They each exposit their various activities. Minthara is not here because she is only present if she is partnered with the player (she is still a speaker here but can change).
END_GameFinale_WithersGathersCompanions
6 months later after saving Baldur's Gate, Withers gathers the companions and player around a campfire to invite them to recollect what they've been doing.
END_GodsAndMonsters_WithersChronicles
Scene that plays after END_GodsAndMonsters_WithersMeeting in which Withers settles the accounts of what happened to all the characters after the events of the game ended.
These files have tons of variations in them and are overly long so I will not copy everything that's in them here, but in my opinion it can be assumed that these are the "17000 endings" Larian talked about just before release. Maybe the game was shipped with everything leading up to the ending finished so they could get it out the door and had time to add all of this stuff later, but nevertheless I feel people should know that these files exist and that there is already a major framework for an epilogue, ending slides etc.
If you want to take a look yourself, go to the link above. There is so much in there that it's hard to cover everything at once, but my personal favourites so far are Karlach having her ending slides take place in the City of the Dead if she dies at the end while playing as her and Withers repeating the question you asked him at the beginning back to you at the end. It's such a shame this is not in the game already.
link
https://www.reddit.com/r/BaldursGate3/comments/15umpxk/to_those_disappointed_in_the_endings/