> I still don't see how any one entity could pull off 20-30% mining power, but I don't have a ton of knowledge on this.

If you were a miner today, you make money when you add a block, and you are awarded 6.25 BTC if you are lucky enough to create one. All the miners have the same pool of transactions, and all of them are in competition to create blocks (lots of SHA-256 hashing to find a random hash value that is small enough).

A new bitcoin block currently averages a bit over 4000 transactions, per ycharts.com - Bitcoin average transactions per block. And a bitcoin transaction has an input amount and one or more output amounts. The inputs do not have to equal the outputs, and the remainder is claimed by the miner as a transaction fee, in addition to the 6.25 BTC payout.

Since all miners have access to the pool of transactions, they can give preference to minting blocks containing transactions with the largest fees. If you leave a small tip, you might not have your transaction processed for a while. But if the miner gets 6.25 BTC for minting the block, that's $350K USD equivalent today. That is much more than the transaction fee revenue, ycharts.com - Bitcoin average transaction fee, which is a $6k USD at $1.50 per transaction.

Over time, the mining reward halves in a predictable way that the Bitcoin system self-regulates, to a degree, and in 2028 it will be reduced to 3.125 BTC. That will decrease until all 21M BTC are mined in 2140, at which point it will be zero.

If your cost of mining hardware and energy is profitable today, you have to either hope BTC prices double by 2028 when the payout reduces, or make additional money on transaction fees to still break even. And you have to hope BTC keeps increasing over time, or continue to make up for it in transaction fees.

However, if there's more efficient mining operators as the payout decreases, they are going to be able to profitably take the lowest transaction fees which might not be economical to smaller miners. Over time, that's a dynamic that leads to a concentration of mining power, and to my analysis, structurally increases the history rewrite attack vulnerability in a way that's unavoidable.

I just read up on this for a job, though. Hopefully someone out there on the boards knows more than myself and can correct me.