Not quite. FP is managed througput. The same number of people get to ride an any given day, it's just that a large % of them are managed via FP. You keep saying "laws of math" but there's a whole science around this called queueing theory that describes all this, and a managed line allows the same throughput than an unmanaged line while making wait times far more manageable.
The ideal scenario from a pure line management perspective would be to have 100% FastPass. Then everyone would wait for a very small amount of time. But then you would need to plan every attraction, which also wouldn't be ideal. I think the 85/25 split makes sense given what they are trying to achieve.
The real issue isn't FP - FP is needed given the current state of the parks unless they limit attendance (as they have been doing since COVID) or build more capacity. As it is, there are more people in the parks than can get on any one attraction and there aren't enough attractions to make the wait times short. So FP is necessary to distribute and manage the load.