This was a frequent excuse back when SpaceX had just started landing Falcons, but both factors go hand-in-hand. If you have reuse, you can fly a bunch of missions.
SLS is going to have difficulty maintaining a 1 launch per year even without payload readiness, so of course it's expensive and therefore disposable. But if they had reusability, they might have been able to fly it more often, thus helping make it cheaper on average to fly.
With SLS this becomes stark in the sense that if SLS was reusable, it could fly more often and would have lower per launch costs, which would make the other lunar lander proposals more viable and would make it more realistic to fly science payloads on it. If SLS were reusable, establishing a long term lunar presence with it wouldn't be a joke that everyone pretends to take seriously because it wouldn't be a matter of choosing between sending over a crew or launching components for a base for a given year.
> But if they had reusability, they might have been able to fly it more often, thus helping make it cheaper on average to fly.
This is true, but the issue is that there just isnt the need for it really. Very, very few missions actually require a SHLV. There just arent enough payloads atm, and that probably wouldnt change a ton even if the rocket was cheaper
Plenty of the things needed for establishing the sustainable long-term presence on the Moon that Artemis claims to aim for would benefit immensely from a cheaper SHLV that can fly more often. That's entirely why Starship ended up being such a valuable deal for NASA. Even the NASA OIG has been saying for a few years now that SLS with its current cost structure and flight rate is not compatible with Artemis's goal of being sustainable.
SLS could absolutely be cheaper, but reuse isnt really part of that equation.