You’re asking why it’s not possible, and then explained a work around that would be needed because it’s not possible.

What you’re suggesting is that instead of selling a service to you based on a quantifiable number based on things within their control that you can reasonably hold them accountable for, they should sell their service to you based on a series of educated guesses about what you could potentially get based on a number of factors entirely outside of their control.

Accurate enough for who? It wouldn’t be accurate enough for me. And it certainly wouldn’t be accurate enough for the consumer who you described as expecting that estimate to match reality. And worst of all, it wouldn’t be enough for any consumer protection agencies or remedies. So many assumptions would have to be made based on factors entirely outside of their control in order to sell to you based on your file download speed. And if you allow them to sell you a product based on assumptions of things outside of their control, then you then have no recourse to hold them accountable. I don’t want companies to sell me a product based on a number offered via a shrug. I want a promised number I can measure against what I’m getting.

Transmission rate is the only metric with any meaning in this context, because the transmission rate is the product they’re selling to you. The overprovisioning and etc is actually accounted for in the transmission rate they’re selling you. Thats why they almost always overprovision your connection based on what you’re paying for.

It’s as if you’re asking for a car dealer to sell you vehicles based on how many how many people you can move between two locations per hour, instead of the number of seats it has and the speed it can go. They would have to start making guesses based on the average waistline of people in your area, the speed limits of the roads in your town, your driving ability, what routes you like to take, etc. that might be sorta handy because you don’t have to do that math yourself now, but it’s not going to be all that accurate to you specifically, and if that number of people is lower than what they estimated, it’s kinda on you because they don’t know if your friends are petite or 600 lbs, etc etc. But if they sell you a car that seats 5 people of normal healthy size and it can go 100mph, and they deliver you something that seats 4 with a top speed of 80, they’re accountable for that.