Red Dead Redemption 2 PC launch was rocky but fans hoped the issues would be resolved in the month leading up to Steam release. They weren't and it refuses to boot or even crashes on this platform as well, with controllers being pointed at as the main culprits.
Rockstar Games and bad PC launches seem to be becoming synonymous. Red Dead Redemption 2's first launch was plagued by failure to launch, crashes and more problems caused by Rockstar Launcher. The "second" launch is basically the release on Steam, which was put on a month-long hold since the release on other PC platforms.
While we didn't come across any fixes or workarounds for that particular matter, the crashes and failures to boot via Steam can be bypassed in some cases. Namely, a different player that plugging a controller in your PC can mess with Red Dead Redemption 2's stability. It seems so far that unplugging controllers from PC can solve the issues of both failure to boot and crashing.
Considering that it's not Steam-wide issues and other games run fine with controllers, this problem is most likely on Rockstar's end. The initial PC launch problems have been largely resolved by now so it's to be expected Steam-specific hotfixes are coming soon as well. Until then, try sticking to mouse and keyboard while keeping any sort of controller unplugged.
Meanwhile, Rockstar's reputation regarding online experiences of their open-world games is not exactly stellar and fans are already starting to on that. Let's hope we don't start seeing horse-mounted Gatling guns as well.