I 100% support this proposal. How it will be implemented is another question.
I would propose even going a step further and having the protocol automatically assign keys, distributed evenly amongst all eligible validators (that fall within a secure range) before assigning ‘extra’ keys.
Something akin to a round robin allocation, cycling through until the number of keys are exhausted.
This is of course an over-simplification but I hope you get the idea.
And of course, with the current disproportionate key bids amongst shards may also present challenges.