Never said it was locked. But being locked or unlocked doesn't change the motivation and is irrelevant to my point. The more people on the steam ecosystem, the better for them. It is far easier to use steam on the steam deck than any of the options you put forward. Even so, not preventing their use means people are more likely to buy the steam deck, which in turn means they are more likely to use steam as their primary store.

Epic, etc are not a default and at the beginning were not easy to get working.

Oh and you got it the wrong. Those things support the steam deck (usually via third party hacks), the steam deck doesn't support them, it you would be able to install them from the steam store with tight integration into the library by default. It just doesn't prevent them. That is a BIG difference. But yes, doing similar things on other hardware might not be possible or at least as easy. I'm grateful their are so many enthusiasts out there making such things possible.