Why should there be a maximum? Why should a community be forced to cut back spending on something they want to? A floor levels the field for all entered bands and forces communities to support activities that exist in the community. A maximum forces communities into not putting forth their best effort. What if they are a band community and value band and the arts over ag or football? Do they have to bend to your will just so the band has a maximum amount of money even though their school prefers to focus on marching band in order to compete at the state and national level in competitions?

Your argument when turned around is just as specious.

For that matter, there are PLENTY of ways for lower SES programs to fund private lessons. Fundraising. Grants. DonorsChoose. BigThought. It can be done. It's just that very few people who work in those communities have the experience (or the time, or the commitment, for that matter) necessary to know how to engage those parts of the community that would support it. I've been in inner city schools for over a decade now.
The average band director in my district lasts 3 years maximum.

Additionally, very few parents see the value in taking private lessons to get better at playing an instrument the kid likely doesn't even own. This is why equity is important to the discussion. A rising tide raises all ships.

And finally - an otherwise medium to large band with three tubas can supplement their bass sound with synth. In this day and age, there's no excuse for a band to not sound polished and of high quality if they want to do so. You can complain about the cost - but if I were in an ostensibly competitive 6A program and I only had 3 tubas in my marching band, I'd first ask the middle school directors just what the hell is going on down there, and then second I'd budget for something to supplement my band sound until that problem is fixed.

Stop looking to why it can't be done and start finding solutions. So what if you can't keep up with Highland Park's or Friendswood's average parent income - literally no one can (those are the two richest ISDs per capita in Texas, according to TEA). That doesn't mean that you can't beat them where it matters - on the field. Teach.

Postscript: Since you started this by claiming that no one can compete with a particular program that has 50-some-odd names listed on their website .. how did they wind up doing at SMBC? Did they win?