Meta doesn't need to dominate the market. Being the Android to Apple's iOS or the Windows to Apple's macOS is way more than fine for Meta's business. iPhone's did $250b in revenue in 2022. Meta as a whole did $117b.
They aren't looking to replace Apple, they are just trying to find a way to get their revenue growing again. It's a huge win in their eye's if Quest3 contributes reasonably well to bringing their 2023 revenue to about $130b.
If there was a genuine third company competing with Meta then that would be a problem for them. Valve and Pico are roughly 0% the size of Meta and Apple. They just won't have the resources to actually compete. Google has been rumored to be working on a headset. If that's the case and they didn't fire that entire org then that would be Meta's only real competition for second place.