They won't do this, because starting out as non-free doesn't get the same traction. Now that I think about it, it's basically a form of enshitification: draw people in with one deal, then after they're onboard, change it to something less favorable.

Why is it enshittification if you only have to pay when you have >$5M in revenue? Is your world so shit if you cross that threshold?

I think the issue is more on how you audit such things. Governments struggle (partially by design, but still) to get proper audits on who uses what and how much money they make.

The theory is great, but I'm unsure how you execute this.

Well, that's even more reason to not complain about it.