First, cost optimization directly contributes to how much markup they can charge. Say on average they deliver savings of 20%, that is excess value OR/Stripe can mostly capture.
Also, you don't need to mark up tokens much if you're a commodity volume business. Think of Costco and their margins & membership fees. Not everything has to be high margin, not everything has to be a SaaS subscription.
Well, it depends on how they deliver that savings. If it’s through something unique and defensible, something that only they (with some moat) can do, then yes, they can argue for a fraction of the savings. But if prices just go lower because things are commodities, then they are going to be viewed as middleman markup on top of already low prices. In that case, their value add is considerably lower.
Also, you don't need to mark up tokens much if you're a commodity volume business. Think of Costco and their margins & membership fees. Not everything has to be high margin, not everything has to be a SaaS subscription.