I'm certain it won't be a problem past a month. Primary input on PC is mouse and keyboard. The primary users for this gamepad will be steam machine owners. The controller for now requires steam. Some sunset of ~150 million people will buy this. I'd be surprised if this sold to even 10% in 5 years. It'll be competing with the likes of 8bitdo and others that while they may have less buttons, no trackpad, it's the amount of buttons devs design around. Steam controllers until they become overwhelmingly popular along with other controllers that mimic it, those rear buttons won't be essential for gamepad native games. Steam controllers have nice to haves rather than being the minimum needed. Vast majority of gamers would be served by a $30 8bitdo perfectly well for their uses
I'd expect the PlayStation and Xbox regular first party controllers and maybe even the premium first party consoles controllers to sell more than this and they don't seem to have supply problems. Those are essential to the console experience. Steam controller is not to PC gaming