I see your rationale and it’s a pretty valid one. I wrestle with the same predicament too.
But, unlike you, I’ve been unsuccessful. The thing is that it sounds reasonable but it isn’t rooted in reality.
The clients submitting RFP want to know the overall scale and cost of engagement. May be, they have a budget. May be, they are crunched for time. Whatever their reasons might be, they just need to know up front.
Even though, most of the times estimates end up changing but it’s extremely hard to educate the clients up front.
Overall, great thoughts. Love your writing.