MLS has a monopoly on first division soccer in the United States.

Who doesn't have de facto monopoly on first division soccer? Seriously, name any FIFA nation with two first division soccer leagues. No anti-competition exists, but if you don't believe that then someone should sue them on those grounds and let me know how that works out.

(Canada, Wales are the exemption, not the rule)

Thus, those other clubs can't offer the promise of first division soccer in the future.

That promise, that hope, could attract fans

The other two professional soccer teams in the area averaged 1,750 fans and a couple hundred of fans separately. Unless if you cite your sources, they haven't "promised" anything.

You know why they haven't promised anything? Because they can't afford to play at the higher level and they're fine staying in their lane, for now.

Instead, they are forced by the monopoly to forever market themselves as minor league teams

Those professional soccer teams came BEFORE Charlotte FC. Sorry to hear that there's not enough interest in that area for Charlotte Independence, but MLS isn't keeping them in USL since the two are widely separate organizations. There's a crowded sports landscape in the area, and that's not inherently MLS fault - United States is just unique that there's five major sports leagues.