It would definitely be a technical nightmare, but let’s say for the sake of discussion that it is doable (to have a flexible block reward that pays miners a fixed amount of fiat).
This would free miners from speculation because they would be able to calculate exactly how profitable mining would be. This makes difficulty (and security) completely controllable based on the fixed fiat payment amount.
However, this would mean only the most efficient miners would exist - since they are the only ones who stand to make any money in a scenario where the mining “market” is saturated. This probably leads to centralization around ASICs and ASIC producers.
Doesn’t sound like a good plan, even if it was technically feasible.
So, are we talking about a target supply cap or a target issuance rate with no supply cap?