I'm sure SpaceX would be more than willing to sign a non-exclusive no-minimums fixed-price-per-launch (based on mass/orbit/etc) indefinite contract for any types of missions the SLS could hope to do. This type of contract isn't even new to NASA (or SpaceX), it's just the policy says "competition" like SLS has to exist and be funded too.
The two main stated aims of fostering competition are contingencies for any single provider and hopes that funding competition lowers cost in the long term (which is separate from preventing cost from going up). I used to be much more supportive of this policy... but nowadays I find myself on the fence. It's hard for me to believe however many billions of dollars we funnel into SLS per launch will ever result in cheap alternatives being developed. It may even have the opposite effect of "SLS got funded through all of its overruns on this policy, we should have no problems doing it again". On the contingency side it's a bit harder to navigate... but it's starting to feel like programs like SLS don't produce realistic alternatives anyways so how much of a contingency is it really providing to fund things like that.
The two main stated aims of fostering competition are contingencies for any single provider and hopes that funding competition lowers cost in the long term (which is separate from preventing cost from going up). I used to be much more supportive of this policy... but nowadays I find myself on the fence. It's hard for me to believe however many billions of dollars we funnel into SLS per launch will ever result in cheap alternatives being developed. It may even have the opposite effect of "SLS got funded through all of its overruns on this policy, we should have no problems doing it again". On the contingency side it's a bit harder to navigate... but it's starting to feel like programs like SLS don't produce realistic alternatives anyways so how much of a contingency is it really providing to fund things like that.