At the end of the day this is great news for Valve as they really only make their money on people buying PC games (through Steam). You were in a situation where mobile gaming was really only practical on the Switch or phones, both cases that would result in lost revenue for Valve. I can see the Steam Deck as being intended to kind of kickstart mobile gaming for PCs, in which case these other companies joining is a complete victory for Valve as is it provides more opportunities for consumers to spend on Steam.
At the end of the day this is great news for Valve as they really only make their money on people buying PC games (through Steam). You were in a situation where mobile gaming was really only practical on the Switch or phones, both cases that would result in lost revenue for Valve. I can see the Steam Deck as being intended to kind of kickstart mobile gaming for PCs, in which case these other companies joining is a complete victory for Valve as is it provides more opportunities for consumers to spend on Steam.