My main question is what is the point of loyalty, when in reality, you are tied to a small proportion of properties.
Except, you're not.
I'm Titanium with Marriott. When I look at hotels, I usually start there, but I'll check other places as well. If one of those other options is a clear winner, I'll book it. If it's close, I'll factor in the benefits I get. Sometimes, even with benefits, a non-Marriott property wins out.
Sometimes, the deciding factor is redeeming points. When it's not, those points are still there for a later use.
As far as I know, no one's ever been turned away from a non-Marriott property due to their loyalty to Marriott, so no one's really "tied" to it.