ISTM that the capital gains tax rate may be irrelevant to the OP’s hypothetical (a small investment grows to billions in a retirement account). If the account is a Roth IRA, the investor paid taxes on the income before it was invested, and the earnings will be tax-free no matter how large they grow.
But count me among those who believe having a separate, lower tax rate for capital gains is highly regressive and increases wealth disparity. (I even started a thread on the topic a few years back.) If the OP’s billionaire buddy had their bitcoin investment in a conventional IRA, and thus had to pay taxes on it when it was withdrawn, there’s no earthly reason why they should have a lower tax rate on those billions than I pay on the money I earn.