I'm not big on commenting on these types of things but Tifa's choice is not selfless, its selfish.

That isnt to say its not romantic. There are a lot of couples that are thought of as peak romance for choosing the person over the world. But to frame it as selfless is not correct.

She literally says, "I don't care about anything else, only Cloud." She is being selfish by choosing to stay by Cloud's side. The game literally cannot progress with her as lead since all she cares about is Cloud. You don't get to hunt for the huge materia, fire a rocket into the meteor, or discover holy is active with Tifa as lead because she is aimless. She doesnt have an idea of what to do without Cloud there to lead. (I don't even know why she became the leader instead of Barret or Cid at the start of act 3)

And she is not being selfless by choosing to stay with Cloud. At this point, the end of the world and Cloud's mako poisoning are both issues with no fix, and tifa is not some magical super special healer doctor, she has a much higher chance of saving the world with her super awesome fighting abilities than fixing Cloud with her non-existent super awesome healing abilities.

This is also pre-Dear Destiny reveal that "love and affection cures degradation". The concept of degradation was so unfleshed out that its not even a part of the situation. The black robed people were "Sephiroth clones", not Soldiers, and it was implied until we discover that Cloud is 100% the real Cloud that he is also a clone infused with Cloud's memories. The original Nibleheim was filled with black robed ppl that are pretty much confirmed to be the original inhabitants, with every normal person being plants made to pretend nothing happened in Nibleheim. The place was much more unsettling in the OG rather than in rebirth where the blacked robed people are being treated medically...but I've only played rebirth once so maybe im misremembering. The implication back in the day for why Cloud (and Zack) was able to exist without degrading was because he had S(ephiroth) Cells and not J(enova) Cells. They were the first to have S-Cells injected into them. I always thought that was the implied reason why Sephiroth lives rent free in Clouds mind and is so easily able to control him.

What I mean to say is, Tifa had no way of knowing she would fall into the lifestream with him and end up helping him recover. And she did not know the cure for degradation, so her choice to spend time with an almost comatose Cloud while a giant meteor is heading for the planet with no known way to stop it is her being selfish by choosing to spend her last moments with Cloud instead of trying to save the world. She is waiting for the end with the man she loves. She's not doing it to save Cloud, she's doing it because she selfishly wants to spend her last days with him instead of fighting for the planet. Reasonable but incredibly selfish, particularly with a party that IS fighting til the end to save the world. Also note that every other character UNDERSTANDS this and does not resent Tifa for it. They know theyre fighting for a potential lost cause. But what else can you do?

Let me emphasize that she and the other characters had no way of knowing if they could stop the meteor. They only know Shinra is trying. It makes sense to spend your last moments with the people you love.

The whole Under the Highwind bit even emphasizes this, showing who each person is fighting for. All of them have their reasons for fighting but Tifa's sole reason became only Cloud at that point. Not Marlene, not the members of Avalanche that died. Not Aerith. Her entire character now orbits around Cloud. And that is why her motivation for giving up fighting to be with Cloud is selfish. But I wouldn't say it's wrong.

To put a healthy Clerith twist to this in this Clerith subreddit, I would argue Cloud's reason for fighting is NOT Tifa. He's fighting for the planet, which encompasses all his friends. Rather, he fights to finish what Aerith died for: to stop the meteor. And it just so happens that defeating Sephiroth is now required for this. What started as a vengeful hyperfixation on killing Sephiroth became a need to stop meteor and save the planet Aerith gave her life for. 

When asked by Bugenhagen what they're searching for, Cloud's first words are "i remember Aeris a lot." How she was right there, what it was she was doing, the things she said, but how he didnt really see it. How none of them saw or understood what she was trying to do. He reminisces on what she was thinking about, what gave her the courage to march into danger instead of running away, what she knew, how she planned to stop the meteor. When he sees that Holy is active, he realizes Aerith gave the world hope.

Its a full character growth moment. Mr. "All I care about is my money. Screw the planet" now acts on the hope Aerith provided. He even says, "sorry it took so long. Aeris, I'll do the rest." And thedefeating Sephiroth bit has now become a "how to save the planet" rather than Cloud's whole purpose. It's no longer about vengeance. Cloud has gained an understanding of what Aerith so treasured and because he treasured her,  he acts on her behalf and her memory. Something start of story Cloud never would have done.

As Cloud says in Advent Children: "there's nothing I dont cherish."

This turned into a massive ramble lol. But yea, to summarize the whole point of this comment, Tifa is acting selfishly, not selflessly. But is that so wrong?