I think your logic is sound, but the problem with the low end approach is persuading developers to migrate quickly. Apple strategy may depend on chip availability. If they can build a lot of high end machines in 2021 then they may go higher end in November.
Yeah, but I think they've made that easy enough with their tools and the message at WWDC was pretty clear: no point in holding back, we're going to ARM. Some devs may not choose to update right away, but I think most are naturally forward looking and doubt many want to be seen as slow to adapt.
The chip availability question is on my mind too. Even Apple can't do everything it wants to at once-- they have a lot of but still a limited number of skilled people to work on these designs and the more things they change at once the greater the risk.
If it were my program, I'd consider a process like this: get MacOS running on an iPad, build an iPad chip into a Mac form factor and peripheral set (my understanding is the dev kit hasn't fully delivered on this yet), maybe tweak the iPad chips a bit to make them a better bridge to Macs, build a Mac specific chip variant. I don't know if that's the process they have in mind or not, and if so I don't know how far along they are. They might have been moving along this path in the lab before releasing anything to the public.
Designing, bringing up and then optimizing a new processor of this complexity isn't trivial though. It'll take years from whenever they started to get it dialed in. The've already had a steady pipeline of iPad chips in development, so it makes sense to tap that stream of parts first.
I could be wrong on all of this, but there might be different answers depending on whether you look at this through a marketing or engineering lens. Sometimes Apple gives the preference to the marketing view by running the engineering cycles entirely in the lab out of public view because they have the money to do that, sometimes they make incremental releases like a normal company that needs to maintain a revenue stream to justify a development and wants early customer repsonse to steer future products.