The test that actually decides it
A partial renovation works when what stays is sound and what goes is the problem. It fails when the two are connected — replacing a floor around a vanity that will be replaced next year means paying to cut in twice.
What makes a partial job cost more than a full one
Matching. Working around fixtures that stay costs time, and an exact match to existing tile, flooring or trim is rarely available. Both are avoidable in a full renovation and unavoidable in a partial one.
When a partial update is clearly right
When the underlying structure and waterproofing are sound, the layout works, and what you dislike is on the surface.