They absolutely would oppose them and create (or exclusively use if they don't already have it) their own platforms. Normies will buy their ball&gun games where they are available. They probably don't know that Steam has anything other than FIFA or whatever it's called now.

Losing AAA games would destroy Steam. The revenue of only indie and AA games is only a small (albeit significant) portion of the total. Even the only remaining AAA games in this hypothetical scenario, Valve's own games, would not save them.

Besides, this isn't feasible. Either they ask publishers to remove the DRM or leave (they will leave), or they forcibly push updates with cracked executables, which is illegal and a violation of trust.

If it's successful it might under cut the claims against Valve but they can just claim the press from the lawsuit helped break the monopoly -- and besides the lawsuit was just for PR and to try to get Valve to break financially. It was always a longshot that it'd do anything else. If it's unsuccessful, however, it can be blamed entirely on Valve and can be used to get the lawsuit to actually do something as "look we implemented everything Steam did and didn't get any more market share."

But Epic kinda doesn't have a choice. They're bleeding money, Fortnite is waning, they have no popular properties on the horizon, they're hard into AI for customer service and moderation which is only getting more expensive, and they keep doing the sweetheart deals to devs for their game giveaways which haven't really brought any paying users to the platform.

It's either attempt feature parity or start cutting the consumer facing part of the business, and for one of the industry's oldest and most prominent players that's really not a choice they want to make.