> 21m cap when the end of Bitcoin rewards start looming.
This has actually already happened with Monero. The mining reward schedule was much more aggressive in it's diminishing returns. Once the mining returns cross a threshold of not being able to sustain the cost of mining things dried up very quickly. Devs jumped in and added Tail Emission [1] so that all blocks have a fixed reward of 0.6 XMR that will never change or go away.
This has actually already happened with Monero. The mining reward schedule was much more aggressive in it's diminishing returns. Once the mining returns cross a threshold of not being able to sustain the cost of mining things dried up very quickly. Devs jumped in and added Tail Emission [1] so that all blocks have a fixed reward of 0.6 XMR that will never change or go away.
[1]: https://www.getmonero.org/resources/moneropedia/tail-emissio...