They did it with the iPod to iPhone, for a famous example.
Jobs wanted the iPad to represent a unique interface and experience that was distinct from a laptop. They failed to do that and the touchscreen is the only difference that remains. Merging these products is only likely if they doesn't reduce overall expenditure on Apple products in the long-term.
Who said that? In my experience they succeeded. That's also why it's the main tablet that ever sold shitloads and still does after a decade...
If you look at the development of the iPad UI over the past 2-3 years it's clearly converging more and more with laptops, rather than trying to differentiate itself. The differences between a 12.9 inch iPad Pro with a Magic Keyboard and a MacBook Air are getting fewer and fewer with each OS update.
Merging 2 usages into a single experience = you flop like Win8/Win11/Gnome/Unity8
On other hand Android/ChromeOS and macOS/iOS/iPadOS is a big winner
I wouldn't expect to see this on the Air or even Pro models as standard; it makes sense as an expensive upgrade akin to the M1 Max.