I want to preface this by noting that I'm starting from two assumptions:
1) Forms of continued existence that don't include some level of personal consciousness are to me equivalent to non-existence. It can be comforting to think of becoming part of the Earth, or continuing to affect others, but it is still the end of what meaningfully makes you "you."
2) Based on what was established last week, there's not a lot of room, IMO, for looking at the door as anything but a means of ending your individual existence. This isn't a case of "we don't know what's out there/death is the next great adventure." A group of people and entities decided that eternal life was driving people mad, and gave them an exit door.
So to me, acceptance of the finale predicated on either the idea that the gang would be living on in some meaningful way or on the idea that the door wasn't really the end falls flat.
Given that I don't believe those things, to me, this ending--three of our four deciding to walk though the door--was bound to be either desperately sad or woefully unconvincing.
Possibility 1: Life becomes so unbearable for the characters, after a given number of bearimies, that they choose non-existence to end their (after)lives of torment.
Possibility 2: Life is still pretty damn good in TGP, and the characters nonetheless choose to irrevocably and eternally end their existences in preference to remaining around with their loved ones in Paradise.
Obviously, we got possibility two - and so for me, it was unconvincing.
First of all, I'm not actually convinced these characters were "done," except in a fairly shallow sense of that word. One of the most exciting things, to me, about TGP is the idea that you could try out other types of roles and lives -- once you got tired of those things that you loved most on Earth, you could embark on any number of other experiences that would never have been available or immediately appealing to you and maybe never even existed while you were on Earth. Other than Tahani, who notably is the only one who doesn't choose to go through the door, it really didn't play out that way at all. It seems like Chidi, for instance, spent time reading and teaching great works of philosophy, taking part in cultural experiences that would have appealed to him on Earth, and spending time with loved ones. But that strikes me as a pretty narrow use of unlimited opportunity. Okay, maybe Chidi wouldn't have been satisfied moving on to reading trash. But what about approaching the mysteries of the universe through becoming a master physicist, as well as a philosopher? What about writing a novel? Becoming an explorer, now that he is free of all the terrible anxieties that dogged him during life? And you know what, if he did want to spend a few Bearimies luxuriating in nonsense - well, there's a reason people have guilty pleasures, and in an eternal world, there's really nothing wrong with just spending a ton of time on pure pleasure. But once that wore thin, there would still be the opportunity to put aside personal pleasure and live for helping others.
Would that be satisfying for eternity? I don't know, frankly; the idea of eternal experience is inconceivable to me. But what I saw on screen were characters who still seemed pretty meaningfully similar to the ones we had known all along, and who didn't seem to have exhausted the nearly infinite possibilities available to them.
And clearly, Jason, Chidi and Eleanor weren't supposed to be miserable. They were fulfilled, and maybe couldn't enjoy or appreciate things as intensely as they had before that moment arrived. But...why would eternal non-existence be in any way preferable to a vaguely pleasant if no longer entirely satisfying existence in paradise? Jason enjoyed his dance party. Chidi did like going to Rome and Paris, and spending time with Eleanor. Eleanor felt good about helping people. So why is ending everything, forever, better than that? So again, it comes back to the central contradiction: eternal nothingness makes sense if they are actually unhappy, in which case their choice is sad and does indeed resemble human suicide. But it really doesn't make sense if they are happy but feel "complete." Ceasing to exist is not a form of ultimate self-actualization. It is annihilation.
So what they give us, as far as I'm concerned, isn't akin to suicide, since it isn't out of despair - but if it isn't out of despair, it is very hard to see why this is the right or a sensible choice.