But it is analogous to the situation I raised earlier for dual citizens - a dual US/EU citizen, travelling with a US passport and a national ID. Currently, that passenger can board in the US with their US passport and use their ID (without passport) to enter Schengen. Once ETIAS is in place, that person presumably would not be allowed to board with only the US passport because there would be no ETIAS associated with that passport.

That is similar to the current situation of a US citizen with Schengen residency - that person would be travelling on a US passport plus another document that gives that person the right to remain more than 90 days.

The airlines might handle both situations the same way, they are both passport + a governmental non-passport document.