1. This is a matter of OS; there are options. Maybe it's best that the device have modes that when plugged in, it uses a partition of MacOS. Or maybe it's expanded such that it uses Stage Manager like iPad, but has a file system more like iPadOS. Or some hybrid. Either way, it's closer to a modern chip than you may realise. It's not going to have a problem with browser tabs. As mentioned, it's not going to have raw horsepower for rendering video the fastest, but that's not the intention. It's not a total replacement, but the question is when does this get good enough to be good enough.
2. Apple has a long history of cannibalising its own products, as well as a history of protecting its products. It depends where you stand as to where you make a case. In Apple's history, both are valid and defendable. The iPad was a great example where many felt that it would cut into iPhone and Mac sales, but it turned out to do neither more than it made, and a category of tablet was established.
3. People said this about cameras. Most people say this about GPS, but I guess not you. People like all-in-one stuff when it's good enough. You don't cut a steak with a Swiss Army, but that doesn't mean it's useless.
It also doesn't mean the end of new products… for example, what if you had a phone that plugged into a laptop like the NexDock, but the phone got a boost of sorts? Doubles its cores, or adds a better GPU when connected?
Either way, it feels like since Covid, we're in a hold waiting for AI, and AI will have to be offloaded to a large degree. So outside of people doing video (they always rush in to these discussions…) most use cases can be handled reasonably well with an A15. If optimised, they'd be fine.
Anyhow, more than one way to go on this. I think there's more money to be made by exploring this option.