That's the thing; there's literally no incentive for RBG to spend more. They invested in the stadium, they invested in big stars like TH14 and Rafa. Go back and look at highlight videos from the TH14 era. We bitch about attendance now, but from the eyeball test, it was much worse back then, even with the stars and the stadium. They cut down on big star spending and we still won 2 shields and were competitive enough. They also got brand exposure via Concacaf Champions League.
What would spending now do for them that wasn't accomplished by spending back then? There is no financial incentive to winning an MLS Cup. I don't think the league generates enough broadcast revenue for the team to say "you know what, let me drop an extra $10M on transfer fees so that I can win MLS Cup", because at the end of the day - they will just be out those $10M. It's pretty clear that their focus now is to go full Salzburg - get young, cheap players, and flip them for a profit. Not to buy a $10M young South American player to flip. Certainly not to buy a 30+ year old international player so that we can win a cup.
The problem seems to be that people are conflating "Red Bull" the global brand, and "Red Bull New York". Yes $10M is a rounding error to "Red Bull" the global brand, but in the real world, it doesn't work that way. "Red Bull New York" has its own budgets, targets, metrics, etc. to hit. And $10M is in fact a big deal for them. If the $10M won't get them a financial return, they won't spend it just to placate our dwindling fan base.
There's a wide range of players between 0 dollars and 10M young South Americans. I'm just saying get more Kakus and, hell, more Verons and Jorgensens at 1-2-3M a pop. You're way more likely to profit on a guy like that with a lot of upside (even if some obviously flop) than an Alex Muyl. Murillo/Taxi/CCJ are more examples of success stories - keep getting more of those guys. The recent pace of 0-1 high upside signings a year won't cut it.
Salzburg is the perfect model, in fact! They've sold players for over 100M in the past 5 years, but they've presumably recruited from beyond the Salzburg neighborhoods where their GM has friends or relatives.