It's a good idea but the Mini isn't really in the same category as far as processing power and memory capacity go. And this extension chassis is ridiculously expensive for no reason. Still, it's a good alternative if/when the Pro goes out.
Yes, as I note elsewhere, what might be nice would be a "MacMini Pro", with appropriate CPUs, RAM slots, reasonable default graphics, and a couple hard drive/SSD slots, in a sleek, compact box with adequate power supply and cooling.
I'm sure Apple could produce extension chassis at a lower price point than 3rd parties, if they wanted to, but I doubt they want to get into that business.
I think you can daisy-chain six of these Thunderbolt-PCIe boxes, which would give you, what 36 monitors?
And not thunderbolt monitors, they'd be whatever the gfx cards you use support, so probably DVI or HDMI or DisplayPort.