Temporary setbacks. 'Mature themes' is so generic it can encompass anything with violence, horror, drug use, mental anguish, whatever. Most of their big sellers have one of those, shooter games can easily be cut out especially with payment processors hate boner for the 2a, thus guns, extending to even fictional depictions. Valve is losing money on this jewery, big time, and I'm sure aren't just sitting on their laurels. Without games being able to enter early access, they're missing out on a lot of cash they'd otherwise be making.
This kind of ban, if it doesn't end up just applying to pornshit as I assume it does right now, and extended to actual complete games, would include their own titles, half life, counter strike, etc. They'd have to see this as an existential problem for their company.