Thinking about it, Nintendo could market a CF Express ord SD Express cards not as a "cheap memory upgrade" but as a user-replaceable system drive. The system could ship with, lets say, a 128GB card as system drive and a cheap UHS reader. If a game needs the speed of the express drive then it would only allow the installation on the "system drive".
From Nintendo's POV, casuals users would be fine with the included storage + cartridges + cheap SD cards and Pro users would have the option to upgrading the system drive to whatever they are willing to spend. And this upgrade would be akin of changing the system drive on the Steam Deck and Ally, so a high price is expected already.
The main drawbacks would be security, but this might be solved by having the OS on a different drive, and of course cost of having yet another card reader, the system card and and the cost of an additional OS drive if they go for that option.
What do you guys think?