I was also interested in the keyboard, and I think they are either identical-feeling or nearly-so across all three models. The new MBPs could be a tiny bit more resistant, but that could be my imagination (or the fact that the other demo machines have been in use for months, and therefore a little less springy).
I didn't notice a huge difference in screen brightness/resolution/responsiveness, though I wasn't looking that hard.
For me, the biggest upgrade in the new MBPs is the ports, since I'm on a 2-port MBP from 2017. If I could get SD/HDMI/MagSafe on an MBA or the 13" MBP, I would do so in a heartbeat. The speed of the new MBPs is amazing, but I (a non-technical founder) could easily get by on an M1 MBA.
I talked with a knowledgeable Apple employee, and he said that he expects they'll keep the 13" MBP around, and that when it updates it should get the same ports (and definitely MagSafe, which effectively gives you another port) as the new MBPs. He didn't expect the MBAs to get anything other than MagSafe.
My plan for now is to try to hold out until the 13" MBP is updated and get one of those. I prefer the smaller size, and the cost of the new MBPs is just not justifiable for me. I might even get an MBA if it gets MagSafe + 2 USB-C. One interesting tidbit is that Apple's tech specs show longer battery life on web browsing for the MBA than the MBPs. That metric is a relevant one for me. AppleCare is also significantly cheaper on the MBAs than MBPs.
It's possible the Apple employee knows something we don't, but it doesn't make sense to keep it around since there is nothing particularly Pro about it when comparing it to the new machines.
It is possible that they'll get rid of it and make the MBP line truly 'Pro'-only. But I hope not, especially if that means that you can't get more ports/connectivity without dropping 2 grand on a laptop...
Maybe it was that way to get Retina displays to more people. It wasn't possible to package Retina on an MBA with onboard GPU with lower thermal dissipation (1st retina MBA only came out in ~2019 with horrible thermal issues, and also the 12" MacBook failure).
I wish Apple would instead transition the cheap MBP to an MBA+ or high spec MBA instead. More ports, more display capabilities, but not far more GPU or CPU power. The base M1 is fine power wise. Just more RAM, and more usability in port selection. Or even a 15" MBA using previous-gen's MBP LCD panels.