I'd buy a big honking Mac Pro off of the refurb store. Something much bigger than I currently need, with the intention of keeping it running and current for 5 years. My 2008 Octo core MP is pushing 3 years, and has least 1 or 2 years of life in it.... at least.
At the end of the 5 years (or so) I'd be able to reassess the situation. Who knows what will be "current" by then. Maybe the low-end Mini will have evolved to be equivalent to our 12 MPs of today? It's possible.
By sticking with a MP for 5 years I'd have saved all the trouble of migrating to a new platform (and perhaps a new OS). By not moving to a PC I'd have saved the trouble (and cross-licensing fees) of moving to a new OS.
I don't see this hypothetical situation as being such a big deal.... at least for me. Buying 5 years time puts us into a whole new paradigm.
For what it's worth, I think we have at least one more release of a new MP based on today's MP form-factor. It'll have current CPUs, maxing at 12 or 16 cores. Lots of RAM potential. Mediocre GPU. And Thunderbolt baked in.... no more FW 400, and maybe no more FW 800 either. Plus, you will be able to add TB PCI cards. Consider this the transitional MP that will tide Apple over for a year or two while they rethink the desktop computer.
Imagine a modular system. You buy the CPU/RAM unit (looks sorta like a Mini) to suit your needs, and then all of the other parts just snap together using TB connectors - like Lego bricks. If you needed more HDD space you would just snap a HDD unit onto the top of the pile, and the integrated connectors would link it up... i.e. no cables. Other resources, that would normally be placed inside the MP case, would instead come into these modules that you just place on top of your existing modules, using the integrated TB connectors.
It could happen, eh?