I assume you're being serious here, so I'll attempt to be serious with my response. The UEFA Champions League is only for European teams, which I assume you know, which makes this argument even more difficult to understand. America is not in Europe, so the hypothetical argument ends there, but we'll pretend it continues and look at how bad they would get slapped. MLS teams aren't even the best in North America. A MLS side has NEVER beat a Liga MX side in the CONCACAF CL final. Ever. If they can't beat the top 2-3 Mexican sides how bad would it get in the CL? Then you bring travel into play. European leagues are playing every Saturday/Sunday and CL Tuesday/Wednesay. Flying to the US and back in between is simply not feasible.
Honestly can't believe this argument exists