Under the right circumstances, I do think MLS could try to force the issue and push for a more global Champions League, type competition as a replacement. If the league loosened its salary cap rules and a handful of the top clubs built toward world-class rosters, they could theoretically reach a point where it would be hard to call something a “Champions League” without including them.
The downside, of course, would be the loss of MLS parity. The salary cap would likely need to be largely eliminated, and there’s no realistic way smaller-market teams could spend at the same level as the biggest clubs. MLS would quickly start to resemble a top-heavy league like La Liga.
To answer FootyLovin question about a business case. It may sound far-fetched, but I do think there’s a business case for the biggest clubs to spend aggressively to try to reach that level. In the short term, they would almost certainly operate at a significant loss in order to grow future revenue. What makes that somewhat more plausible right now is the equity side of things. The value of the top MLS clubs has risen rapidly over the past decade, and some are already appearing on lists of the most valuable clubs in the world.
The U.S. is an enormous market with substantial upside in ticket and commercial revenue, and MLS teams have shown they can charge prices that are often far higher than what’s typical in Europe. At some point, the question becomes whether the biggest MLS clubs reach valuations high enough that they’re willing to leverage that equity, spend heavily on their rosters, and try to vault themselves into the tier of the major European clubs, betting that once they’re big enough, the revenues will eventually follow.
I think if they want to add prestige to a "champions league" for us, they need to somehow convince the south american countries to do a joint "americas cup" instead of their copa libertadores. concacaf countries would have their own qualifying rounds before entering the main tournament, much like how UEFA does it, and then the top handful of teams enter against the south american big dogs.
Concacaf just doesn't have the money and quality to command the prestige of a regional championship since it's essentially US vs Mexico every year and Mexico wins most times simply because they spend more.
As you mentioned, if MLS wants to compete against south american big clubs they'd definitely have to loosen the salary cap drastically. We definitely aren't at that phase of growth yet, but I can see a future where this all could happen.