As long as they participate in Steam sales, assuming they're on Steam to begin with, PC games are more convenient to have in a library where I don't have to manually update each game. Valve's not perfect, with its 30% cut of sales being arguably too high (as is the case for all other platforms that defend its use as being an "industry standard"), but given Nintendo's monetization of online gameplay and replacing the Virtual Console system with what is essentially console library rentals, I don't mind putting up with updating Switch ROMs once in a blue moon if it means not supporting anti-consumer practices. Any games I had in my Switch library that are also on Steam I simply repurchased for the sake of convenience, however.
Considering that Valve makes more money per employee than most major tech companies, it definitely seems like it would still be turning a profit if its share of sales were reduced to 15 or 20 percent. Steam's services aren't free; the 30% fee inflates the price of games by 43%. As with any company Valve needs to have a high enough profit margin to cover long-term costs and R&D budgets, but the 30% cut is an outdated industry standard from when server operating costs were substantially higher than today.
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.
My guess is that R&D as well as third-party Steam keys eat into their margins.
It could be more sustainable with this higher fee as well. Valve supports old games for a long time whereas console manufacturers pull the plug 10 years later. You could argue that Microsoft takes only 12%, but Microsoft has the luxury of being able to exit the PC games market at any time, or they can take a loss on it indefinitely. Valve needs to survive off its PC store because it's the only thing they really have