You are only scratching the surface. The real problem is not for mobile to take over, we can do it, but when to consider wifi down. It is all fun and games if your wireless network is connected to good fiber, but it is not always the case. Internet uplink may be high RTT or flaky itself, it may be low speed and someone started torrent client and video call, it may as well be the very same mobile broadband with "unlimited throttled home" plan, wifi AP can be connected to gateway over multiple wireless hops, etc.. Platform always has to question "is it problem with remote end or uplink? is uplink 2 better than uplink 1?"

Network switch comes with non-zero cost that all connections must be treated reset and have to be reestablished, so even if the platform monitors quality of all uplinks, we still need some uplink stickiness to avoid constant switching between uplinks in places with spotty coverage or flaky uplinks. I'm not saying current status quo is the best, but currently we cannot have it work perfectly.