Given they just came out with the new cheese grater that solved problems they reluctantly admitted, I'd be very surprised if they discontinued it after ~1 year of sales. So I think it's going to stick around for a while (they sold the trashcan for 7 years without an update). Hopefully it gets frequent spec bumps like the 1,1 -> 2,1 -> 3,1 -> 4,1.
I think an ARM Mac Pro (if/when it happens) would be quite different from what we've seen before. So how upgradable will it be? By whom? If it has PCIE slots, what vendor will make cards compatible with it?
Sure, the Intel Mac Pro was locked into Intel Processors - but I know where to buy those. Where can I buy Apple's custom ARM processor? Apple won't even sell me a replacement battery for my MacBook Air (they'll install one for me if I send them my laptop, but I can't just walk into my local Apple Store and walk out with a battery to install myself).
There's a lot we don't know, but luckily we only have to wait ~24 more hours to find out.