Why do you think Meta will be the only one making hardware?

There are plenty of head mounted displays that don't have any smarts built in.

You can buy a phone that isn't powered by iOS or Android too, but you probably won't.

If you just want virtual monitors, I think there will be many "dumb" headsets available. If you want a full standalone experience, then sure, maybe not. But, I think those are, potentially, different markets.