The decision really will come down to you buying into the Apple Silicon future or not. Then it's lining up your upgrade cycles with product release cycles in some capacity.
The majority of people could probably sit on their Intel-based machines from 2017+ for another few years without a dire "NEED" to upgrade in the next 6 months. Personally would wait to see what AS Mac Pro brings and then get a clearer roadmap of where your needs fit into the product pipeline and budget. If you're on an MP4,1/MP5,1 or know you'll have a limited budget, the time to seriously evaluate the Studio lineup is now.
You can add Thunderbolt PCIe expansion boxes for relatively cheap and those can (theoretically) be moved from machine to machine and probably continue to serve at least one upgrade cycle, if not more. Most existing eGPU boxes can be repurposed to drive nearly any PCIe card via TB. Costs here are $250-$1000 depending on total slots and power needs, maybe closer to $2K if you really need to drive 6 PCIe cards.
Storage solutions continue to evolve and there are many solutions to fit nearly every need and price level. You can get a 4xNVMe enclosure running directly over TB for as low as $250 if you shop ($350-400 from more reputable brands) and don't want to use a PCIe at all.
What is left that you personally need PCIe slots for with an AS machine? That is the decision most waiting on AS Mac Pro already have their own answers for. Driving legacy products on a future machine is not how I'd make my decision and think it will hold many back.
If you're already pushing the limits as a developer, you really need to be on AS already in some capacity and even the entry-level Studio (with a few smaller BTO upgrades) looks like that price/performance that will slot in nicely for many.
