It already comes with a hefty fee of 30% per sale on the platform. I don't think they can raise that without serious backlash. And there also isn't really a need, Steam prints money. It prints money because it's where users are. Users are there because they like the features. Some good features are only there because of laws (e.g. refunding); Valve can't remove these.

So how would you make the service even more profitable?

Enshittification happens because corporations want (more) money out of a service that built a userbase. These were often running at a loss. To turn a profit, they need to change.

Steam can sell you licenses to games you don't own already. It's up to each publisher. Valve doesn't care, they just deliver.