Not BCH miners, not BTC miners, not Calvin.
The money comes from the block reward thus from inflating the coins of the BCH holders. This inflation is usually used to secure the chain and now users will exchange some of the security for money for development. Miners mine what's profitable, no change for them.
We already have much less hash than BTC. Going from 3% to 2.5% probably won't be that bad. Moving that money for development is probably better.
There's no transparency. The company can be seized. The funds can be stoled by the miners (unlikely I think). It's way better to make some smart contracts.
Fiat is bad because it lets central banks print as much money as they want, thus devaluating people's savings. Bitcoin was born to fix this problem.
Satoshi designed the system such that a colluding majority of miners have the power to change the rules. This could be a problem because all miners want more money so they could collude and inflate the coin. It's not a big concern, because people will never support this, that's why they're in crypto after all.
But here we are, thinking about allowing miners to print coins that go to their fund. You might say "they are not printing more coins, they are just redirecting them" but this is just a small detail that doesn't change things much. This fund might get super important and continue forever. Then security might get lacking and guess what - "let's increase the block reward but keep the fund". It's a slippery slope that leads back to uncontrolled inflation.
It's generally a good idea to put money for development. It's a clever idea to exchange security for that fund. It's a bad idea to allow miners to enforce inflation for their private fund.