A noble idea, DeRigueur, but do you really think it'll fly with the couple? I could be wrong, but sort of get the impression that their attitude is "this is what we're doing - deal with it"
No I don't think it will fly.
Looking at their website they make much of still being Royal despite the lack of title (citing Harry being 6th in the succession) and M being high profile in her own right.
I think they believe they are entitled to security in perpetuity - however much money they make.
But here's the thing - it's the Govt not the RF that will make decisions on security in conjunction with the Met. Its not an emotional decision - it's about budget, risk, reputation and public opinion.
The choice the RF have is not to dictate what this decision is but rather to state their opinion and then potentially fund and top up needed.
In other words it a political decision now that sets (as discussed) a significant precedent.
What leverage do H&M have in that context? They can't blackmail Gove and Johnson with a threat to speak to Oprah...
The only thing they can say is "how would it look if you don't provide security and something happens to us - do you want to risk it?".
Of course the answer is no, BUT if you provide them with the £3m for security they currently get and then THEY make choices that comprise their own security by not budgeting accordingly then any repercussions are theirs to own.
So I think if they choose to the Govt could absolutely take that position regardless on what will "fly" with the couple themselves.