I reckon you’d have to be at least in the double digits for larger studios/orgs to take notice. Though the steam deck has already had a very positive effect on the number of Linux native games in the indie space.

For something like FIFA & Call of Duty for example?

I imagine somewhere between 8% and 20% just for enabling anticheat support. That would be around 20 million monthly active users.

Some smaller devs are already doing the extra mile and making native ports, like Baldur’s Gate 3 for example.