Have you ever connected an iPad to an external display, mouse, and keyboard? I agree with you that laptop sales would continue, but I bet they would decline. Apple could do the MS Surface much better than MS did.
The areas where the iPad beats the MacBook are still (imo) the same as the day it launched: web browsing, watching video, and reading. Workloads that are heavy on the Apple Pencil too, but that’s a fairly narrow slice compared to anything else you might do on a computer.
I don’t see anything in my use case that I’d gain from a MacBook over this setup.
Let’s just ignore the price comparison though lol