I have never had a work environment ruined by toxic positivity—the normal healthy human reaction to that kind of environment is gallows humor, which hits the sweet spot between acknowledging the problem and showing a willingness to be there with your team.
I'm sure there are people out there who do have a toxic positivity problem, but my own anecdotal experience leads me to prefer to err on the side of rejecting unnecessarily grumpy people, because they tend to more frequently be a problem.
I think that may be a very cultural thing. I love gallows humor (I understand, enjoy, and cultivate it myself), but some cultures don't even understand it.
Toxic positivity in startups means the people not looking at the real issues role play startup while everything crumbles around them.
Maybe it works out in big orgs but if it infects the team of a small org your work environment will be ruined when you are all laid off after months or years of overworking to make a blind optimist happy. Unemployment coincident with burnout is worse than some negative feedback during the process.
What you're describing as toxic positivity is refusing to criticize things as they're happening. That's very different from not badmouthing people and teams behind their backs when they're not around to respond.
Well you are insanely lucky, its the default in most startups and many companies that I have seen consulting, working directly with, or otherwise. Leadership has no strategy, the business is growing or shrinking regardless of their decisions, the rank and file are restless because its obvious they are led by folks who have no idea what's going on, and nobody is allowed to talk about it.
I'm sure there are people out there who do have a toxic positivity problem, but my own anecdotal experience leads me to prefer to err on the side of rejecting unnecessarily grumpy people, because they tend to more frequently be a problem.