Thanks, I will admit the nitty gritty of hardware can get beyond me. That said, I should have been more clear - I was meaning longevity as for the device capabilities and iPadOS support, rather than the actual longevity of the hardware itself.
I guess I should have simplified by asking along the lines of "with stage manager being supported only on iPadOS w/M1 (lets pretend I wrote that a month ago), is it worth investing in an iPad Pro with an M-series chip over the gen 4 due to better iPadOS support."
Yes, that's an entirely different question. If you're convinced that Stage Manager is the future and will be useful/desirable for your purposes, then Stage Manager compatibility should be part of your purchasing decision, no different than storage capacity, display size, or cellular data might be. It won't be a question of functionality five or more years into the future, but functionality in the nearly-here-and-now.
OSes will continue to evolve. The iOS/ipadOS fork is not yet five years old, and after almost a decade of complaints that Apple was preventing iPad from being all it could be, the tables have turned. We've seen new iPad-unique capabilities nearly every year.
But the future is still unknowable. It's conceivable that in another year Stage Manager might become available on some A-series iPads. Or not. It's also possible that in 3-5 years there will be some new must-have feature that won't be supported by today's M-series systems. It's been that way in computing since well before that day in the early 1970s when I sat down at a TTY terminal in my high school's basement and I committed my first lines of code to a punched paper tape drive.