Your second point is a sound quality discussion. Because an inner city band has more tubas doesn't mean they are playing with the same quality of sound. Show band sound has a tendency to be spread and very bright. That doesn't translate to a corp style sound. BUT if their ensemble sound was the same then yes, that inner city school beats 3 tubas musically.
If parents don't have money to spend on private lessons it doesn't matter how much they support the quality teaching. Schools which require ALL students in the varsity and non-varsity band to take private lessons funded by the individual will have a much deeper level of player than a program where the parents CAN'T afford to spend money each week on lessons. Sure, the district could fund those but that then is a budget decision and probably means something else isn't being funded. If you paid for 20 lessons over the course of a school year for 40 students at $15 per lesson that's costing the school budget $12,000. That's more than my entire show budget including flags, music, and drill....
Why should there be a minimum? jWhy should a community be forced to budget money for something they don't want to. A cap levels the field for those interested in playing. A minimum forces communities into spending they don't want. What if they are an Ag community and value Ag mechs over marching band. Or they value basketball over football? Do they have to bend to your will just so the band has a minimum amount of money even though their school prefers to focus on choir to push budget money towards other programs or expenses like busing or extra teachers?