if hive hits for example 400$ each, witness earnings would allow much more computing power for scaling

Possibly but there can be bottlenecks that are more tied to the software architecture which don't necessarily lend themselves to scaling up by spending money. For example, fastest single-threaded computer you can buy at any price isn't really all that much faster than an iPhone.

Of course, it is also true that with more money, more work can go into optimizing and rearchitecting the software, to a point.

The concept is good, though, but the relationship between money and scaling isn't linear or simple.