That's thw first time I hear an idea like this...sounds interesting
The first idea that came to my mind is the fact that this would eventually change the incentives at the base layer, changing literally the reward scheme. This could be very very disruptive if adopted widely (which I doubt because why donate to miners if you can just can put high fees only when you need the tx to go through?)
Moreover, inventives wpuld change even for future miners. If the block 1'000'000 happens to have a total of 10btc worth of donations (ballpark figure) then miners would be incentivized to try to mine it even when the timechain state passed that blockheight. Imagine miners trying to win that block price and building every one its own fork to invalidate the other's chain ans win the reward of that block.
This is off course an extreme example, nobody really nows what would happen
My guess is that Bitcoin si build on top of protocol rules whereby the future of mining rewards will be one of this two:
  1. Fees increase at the point of replacing the coinbase value. This will lead to a protocol where onchain transactions are only created once in a while by big Layer2 providers (ex. some big LN hubs)
  2. Fees stay relatively low, meaning that the block reward isn't sufficient to sustain the mining industry. This would inevitably lead to a Mining ecosystem where bitcoin-focuses companies allocate some capital for the mining acitiviy, just to sustain the protocol's security and, as a matter of fact, the protocol on top of which their services are build.
The second scenario may be a little dangerous too, because it would mean that the bitcoin protocol is sustained by economical value that 'comes from outside of the protocol', which could undermine the Bitcoin's stabiliy.
Good point regarding the incentives changing, that would not be good
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I wouldn't say that is strictly 'not good', I simply observe that a change like that one would possibly flip a lot of incentives...which is something that we don't want to happen into Bitcoin
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