It's not as simple as that. Android may be open source, but Google can set the terms under which manufacturers can use the Android trademark and sell phones wit

It's not as simple as that. Android may be open source, but Google can set the terms under which manufacturers can use the Android trademark and sell phones with access to the Play store and other Google services. AFAIK they did amend those licenses to contain mandatory updates for some time after a device first hits the market, so it's definitely something they're actively trying to improve. It's probably not the easiest thing to solve, because manufacturers might decide to fork Android if they'd go too far, so they have to keep a balance.