People aren't celebrating WP dying. It's just that the signs have been there for a long time, Microsoft has clearly shown no interest and furthering the platform, and the fanbase has gone to comedically ridiculous levels to defend terrible decisions Microsoft has made. Most people that don't use their devices out of a stockholm syndrome-like sense of need to support a company that's repeatedly screwed their loyal customers over, have long since left. But that basically just leaves behind a group of people who are willing to literally switch banks because the one they were using didn't support a platform with <1% marketshare. People have made Windows Phone their entire identity.
Microsoft made a flagship out of cheap plastic and is charging the same price and MORE in some locations than that of other flagships? Everyone now thinks that plastic is better and comes to the subreddit telling made up stories about how their friends are "so jealous about how light my phone feels in the hand" and raving about how plastic doesn't dent/scratch. But I guarantee if the Surface phone happens, people will do a full 180 and rave about the premium metal.
There's a bug or missing feature? You should upgrade to preview builds on your daily driver.
Preview builds have massive bugs? You shouldn't have upgraded to preview builds.
App is pulled? Didn't need that app anyway. Third party clients are better.
Google isn't making apps for Windows phone? They're being anti-competitive.
Windows phone is dead? No, smartphones are dead!
If WP allows you to customize something, it shows how open the platform is for developers. If WP doesn't allow you to customize something, it doesn't matter because the implementation Microsoft has is the best so there's no need for third parties (e.g. third party keyboard suggestions are shot down any time they're brought up).
I've made comparisons to how much people treat Windows Phone like a religion before, and I see the comparison become more true by the day. People boycott stores because they did something against their religion. They make up stories about their religion. They try to censor people who say bad things about their religion. They practice apologetics and parrot easy to remember lines to make themselves feel better about their religion. They try to convert people to their religion. They project their own practices onto other groups.
People aren't happy to see the platform die. But they're happy to see people finally coming to their senses when as the evidence becomes more undeniable.