Football takes place during the weekend (there may be some overnight travel, but that wouldn't happen until championship week, and schools are usually done for the semester by then), and while SMC takes place during the week (as does C&SR in the Spring), those students are considered to be on a school sponsored function and are counted present for attendance purposes. No attendance dollars are lost. The same would be true if the football team had to travel during the week to get to a playoff site.
The only reason BOA & USBands have a foothold here is due to UIL going to the every-other-year SMC format in 1991. The fact that you suggested that BOA/USB is an acceptable "State" contest in the off UIL year supports the argument for having SMC every year.
Your position regarding the financial expenditures for a state-level marching band doesn't work...again taking into consideration your position that BOA/USB is an acceptable "State" contest in the off-UIL year. There is not going to be any significant difference in expenditures going to a BOA/USB contest as opposed to SMC. Several bands are already doing BOA/USB *and* SMC during the same year already.
Reverting back to an annual SMC format will not affect the concert programs one bit. Bands that consistently appear at SMC are also consistent Sweepstakes bands, and usually participate in Honor Band (which is every other year, BTW). Those programs that get to SMC, but aren't consistent SW bands are the exception rather than the rule. Any director worth their baton knows that if their concert band does not play well, then their marching band will not play well, and they focus on the concert program. The fact that they have an Area and State marching contest does not change that.