Each Artemis launch costs something like $4b (that's the incremental cost of a new rocket, it's much higher if you amortize the design costs).

IMO the program is not optimized for cost or sustainability, it's optimized for creating jobs in various congressional districts. Of course that provides a certain amount of political sustainability to the so-called Senate Launch System.

I just don't see a future where NASA can afford multiple SLS launches per year to maintain a continuous Lunar presence

> Each Artemis launch costs something like $4b

Early launches, yes, because SLS is a garbage heap. Later ones, almost certainly not.