What do 4190024921 and 3652501241 have in common?

Over the first 15 years of Bitcoin, it seems custodial financial services, now ETF issuers and products with KYC are still the most profitable ventures. Is that it? How about bitcoin-only companies? And by "only", I mean: no fiat, no custody of user funds, no verifications. Just bitcoin. Is there a profitable business that can be built? Could that venture even give me a bitcoin-denominated return if I were to invest some of my sats to bootstrap it? I can't just keep mining fiat forever.

I know it exists. I haven't found the idea and execution path, yet.