Aside from hosting cloud saves and Steam workshop data, there aren't many other services that justify a high fee to offset long-term costs. Steam trading cards, for instance, are just another source of revenue for Valve given that they also take a cut of sales from marketplace transactions.

Given that Valve's costs in developing Proton are offset by the higher Steam game purchase rates of Steam Deck users (myself included), the main benefit to developers is Steam's user base. As with Apple and the iOS app store, however, having what amounts to a monopoly in a market segment is not a justification for high platform access fees.