it would be fine to store it in an online wallet
You are going to trust an online wallet for 20 years? Are you insane? Given how frequently third party services, exchanges, wallets, marketplaces, etc. come and go, the chance of any given online wallet still being functional in 20 years is incredibly small. This is a terrible suggestion.
What will the future bring for hardware devices that might be put away for a generation or two so they can benefit from crypto that was passed down to them?
I wouldn't be worrying about the conversion from USB-A to USB-C. You will always be able to find adapters between the two for only a few dollars. What I would be much more worried about is trusting a hardware wallet for 20 years. Most of these devices have barely existed for 5 years. We have no firm evidence to suggest they will still work just fine after 20 years locked in a box somewhere. A bit too much cold or moisture exposure might just been enough to render it useless.
Adam Back recently
tweeted about an absolutely stunning 1000BTC Casascius coin, where his point is pretty much the same. What if the private key is no longer readable? You literally risk 1000BTC, in 10 years most likely worth +$100 million.
Yeah, even if the private key was engraved in steel, I wouldn't want that much money accessible by only one method. Every method can fail. Unless I was planning to sell it, I'd be opening that up and backing up the private key at least twice more.