I’d say it didn’t fail. It was never really a consumer phone. It was an attempt to get hardware in the hands of developers, and it achieved that.
Other posts here discuss why it didn’t receive wider adoption.
I daily drove my PinePhone until I could no longer receive MMS messages, since my service provider has a different APN for the internet and MMS. That, and the modem became more unreliable over time. I like my PinePhone, but an average user would never adopt it as it is.
Likely not. I’ve tried skipping a release once by accident (I didn’t pay enough attention) and it ended with a bricked system and a full reinstall. Don’t do it.