I like the purpose behind this idea, but I don’t think it would work well exactly as stated for one reason:
An Indexer with no open allocations cannot serve queries. This would mean forcing an Indexer to shut down service to change delegation parameters - which is unacceptable for an Indexer providing critical services to dApps.
There might be better ways to achieve the same end. For example, I can imagine that the delegation parameters could be copied at the time the allocation was created. That way the Delegator has the same assurances that their potential rewards will not disappear.