The vendor lock-in scenario for desktop hardware already exists with the latest x86 generation of gaming consoles. Gaming consoles are locked down because the hardware is subsidized with the expectation of revenue from the digital marketplaces they provide.

The yet-to-be-released Steam Machine is not subsidized and is unlocked. Steam is a OS agnostic digital marketplace, so it doesn't matter what OS you install on the machine.

It's just market forces really. In the end, phones provide enough utility for the majority of users while being locked down. There's nothing stopping you from buying a fully-open phone, but there's just very little utility in it for the majority of users.