For game developers, they are a nearing close to a monopoly with Steam, anything PC and for distribution has to go through them and they take a cut, any other places aren't as effective.
Valve has no real competitors or competition.
Unless Valve faces intense competition to change, enshittification will be inevitable since they know nobody can really challenge them.
This is regardless of self or outside funding or no plans to IPO.
Their competition is beholden to quarterly reports, and can't do the sort of long term investments that would be needed to compete effectively with a company like Valve, which isn't beholden to quarterly reports.
Valve doesn't need to enshittify because they aren't subject to shareholders pressuring them increase the profit. As a private company that's pretty much printing money, they don't need to do anything... which has largely been their entire strategy. What would Gabe even do with even more profits, buy a third and fourth yacht?
The need to do 'nothing' works if you are a monopoly or have little to no competition.
Epic and Battle.Net are not real competition.
Do well enough that you don't need outside investors, or if you take on outside investors select them carefully and ensure they don't expect an IPO.