Requiring a review from a trusted contributor ensures that one of those trusted contributors reviews the code. The one main maintainer should add more people to that trusted circle, which will ensure that at least one of those will review all code that goes into the codebase.
If people see that code isn’t being merged, someone will step up to request to be in that trusted circle.
That doesn’t solve the issue that there are too few contributors. Requiring a review doesn’t ensure that someone reviews the code.
Requiring a review from a trusted contributor ensures that one of those trusted contributors reviews the code. The one main maintainer should add more people to that trusted circle, which will ensure that at least one of those will review all code that goes into the codebase.
If people see that code isn’t being merged, someone will step up to request to be in that trusted circle.