Of total users 5% is a substantial number of consumers and some would argue a non-trivial amount of market share to ignore when making a product.
This also goes without saying that the more adoption we see, the better these alternatives will get as we see consumers and businesses view Linux as worth the investment.
Exactly, 5% isn't much, but it's enough to compel developers to make sure their game runs well through Proton, which is all is necessary these days. Ports aren't really worth it, especially if they aren't going to be as well maintained in the long (cough, Valve, cough).
This also goes without saying that the more adoption we see, the better these alternatives will get as we see consumers and businesses view Linux as worth the investment.