It is the Apps, and you perfectly summarized how it's a feedback loop. It wasn't the only reason why Windows phone failed, but it was one of them, even though Microsoft threw money at companies to at least have the most important apps available from pretty much day one. The app store still was pathetic and a lot of basic quality of life apps missing. And they kept going for what, 6 years before they announced they'll scrap it, and kept support going for another 4 years. Now imagine something new comes up, no matter how awesome the system is, no matter how well executed every move of the company behind it is: how much is a smartphone worth that doesn't have WhatsApp, Facebook, Google Maps, YouTube, TikTok, Instagram, you name it. And no, the browser versions don't count.

Heck, you can just use lineage os without the Google services to get a taste of this. And that's a system where you still can get all the Apps with a tiny bit of effort, as long as they don't depend on the Google services. It sucks for the most part.

But really, since you asked, just try it. Get some used phone that's supported by lineage OS and challenge yourself to use it exclusively for a month. Or maybe for fun/hard mode, get a Lumia 950 and try that for a month.