I'm not going to seriously debate the quality of either team - and tbh in my memory the 2018 team was even worse than it actually was - but on 9/1 the Phillies were 2 1/2 games out of the WC (and ahead of every other wild card contender). Last year on 9/1 they were 3 back in the NL East (and had a better record than this year's team). Falling short of your goal in the first year you expected to reach that goal is not in and of itself a fireable offense, especially given all the mitigating circumstances. Collapsing two Septembers in a row might be, but in both cases I think there was a lot of regression to the mean (last year's team was never that good; this year the injuries and Hoskins' slump and Nola's awful finish were big. Some will put those things on the manager and coaching staff of course).
I will grant you the above (and your post) is an additional indictment of the whole operation - while the standings showed a slight improvement and the injuries were a huge factor, at the end of the day the Phillies spent a whole lot of money and changed up a whole lot of players and wound up being worse (than the Braves and Nats) than they were in 2018, and the system doesn't offer more hope than it did two years ago as well. That definitely says fire everyone; not sure it says fire the manager. At the same time, I don't see a path that improves the long-term prognosis - whether they stick with Klentak or bring in a new GM the job will be to double down on the current core for the next two years.
At this point I'm not sure it matters, unless Klentak's job is not actually safe (contrary to the few reports we've gotten). If he keeps Kap and next season fails, he's done. If he gets a new manager and next season fails he's also done. If they bring in Dave Dombrowski or any older, old-school GM, maybe we get a few good years but then it's probably the desert again.
No way they give Kap an extension. As has been pointed out, Charlie didn't get one in 2007 (he also started the season 3-9. Not sure Gabe could survive that). Of course Maddon didn't get one this year and you can certainly argue that didn't help the Cubs' season.