I don’t think they “mindlessly” did anything. They were trying to save the galaxy from Palpatine’s New Order. That’s pretty mindful. There’s only so far you can go in the middle of a war to save as many people as you can from the opposite side during the fighting. That’s part of why war is such an inherently tragic setting. And there are ways to amplify that feeling, or mute it. Star Wars has mostly always chosen to mute it, save for very specific instances, which don’t happen very frequently in the movies themselves (they happen a little more frequently in the Clone Wars show).
My problem isn’t with the characters so much as with the conceptualizing of their behavior and the situations they’re dropped in by the writers. Abrams set up a scenario where Finn defected from the First Order because he’s a good man, and then he just… never followed up on it. Johnson DID follow up on it, and then cut it out of The Last Jedi for the sake of time (I think the minute or two he saved by doing that was ultimately not worth it, and he should have left those beats in). Had he left it in, maybe it would have been so clearly described in the minds of the general audience that Abrams felt he would have HAD to address Finn’s status as a symbol to other troopers that they don’t HAVE to keep working for Kylo and Hux.
Instead he invented Jannah and her team of defectors and… didn’t do anything with them, really. And even then, Jannah’s defection is more character shading than anything else, because the purpose of her was primarily to connect to Lando more than anything else. And even THAT got cut.
Trevorrow’s script (and his ensuing direction) probably would have had their own set of very big and dumb problems, but I think people are reacting as positively as they are not only because it’s forever a hypothetical and the grass stays greener on that side, but because he was actively engaging with those sorts of ideas. Finn’s major purpose isn’t to help Rey find a thing which leads to a thing and then he has to help shut off a thing. Finn’s major purpose is to inspire a rebellion within the First Order. I wish I’d gotten to see that.