You would be hard presses to gather 10000 active Amigians, and only a minority of those run beefed upt "classics". Just ask anybody who actually sells Amiga-HW/SW, and when he managed to sell 1000 units for the last me .....
I'm estimating here, but I've owned 3 mediators (2 1200 versions and an A4000 one) and, having spoken with other people who own mediators and comparing serial numbers, it seems that cards are numbered sequentially, so you can work out how many they sold. When I got my first mediator (about 9 months after they came out) my board was numbered in the 3000 range. When I got another a year later it was around 6500. That was a year ago. My mediator A4000 had a serial number around 350.
So, going from what I can work out, sales figures for the Mediator (all versions) are at least 10,000.
Considering that these people have spent so much money on their Amigas (mediators aren't cheap) it's reasonable to assume that a lot of them would also be willing to spend an extra couple of hundred quid to buy a Shark. Even if only 25% of mediator owners bought a shark that's 2500 copies of Amiga OS 4. This is a big enough market for Hyperion to want OS4 to run on the Shark, and for Elbox to make sure it works.
If and when OS4 does appear and runs on the Shark, I'd expect a surge in people buying mediators and Sharks
I have to say though, I sold my last real high-spec Amiga (A4000 PPC + Mediator) almost exactly a year ago, and I couldn't see myself going back to classic now. It would cost the best part of a grand to get everything needed, and when I can get a higher performing Peg2 for just over £200 with OS included it just isnt worth it. For people who already own mediators though, it's certainly an interesting prospect.