FWIW, it's not like a 7,1 becomes a paperweight overnight. Look how long many people kept getting work done on pimped 5,1 cheese graters. If the delta between how much you can crank out on a MBP vs a 7,1 MP is significant enough to pay for the tower over a couple of years I wouldn't sweat what's next. When the performance delta between the 7,1 Intel MP and an 8,1 ARM MP reaches a certain level then upgrade.
The distance and firmness of that horizon is the issue. I have a 4,1 that has been running the latest os now 3 versions past what Apple officially supports (11+ years). It's mostly because Apple hasn't cut out the CPU I use from being able to boot it. BigSur would never run properly for me if it required AVX. Back on point, the 7,1's will only run the latest os if their CPUs are supported, because the moment they're not, it will be because they're the last to go, and the ARM transition is over. With the ARM transition, the line in the sand will be much more rigid. The PPC->Intel transition from announcement to an Intel-Only mac OS was just over 4 years. After that, you'd get security updates for a year or two, but by that point, nobody will be making new software for it. To keep it running then, you have a machine that only runs heritage software ("legacy" to use the bad word) at best.
I don't think they'll release a proper Mac Pro with ARM until they can arguably compete directly with the old hardware. Otherwise it will be a 2013 Mac Pro, on the order of the Mac Mini that lost half its performance (went from consumer desktop hardware to mobile hardware internally)
All this is a question of how long after Apple's completed the full hardware transition are they willing to maintain the Intel devices? In all likely hood, it will languish on the backend libraries that are being rewritten on ARM, while older devices slowly become excluded from some of the newer features, some bound to hardware (USB5, now with Fiber, aka LightPeak), others bound to software that's a pain to rewrite if they have to (Metal 4, SecurWhozit), or just don't want to.