That's not really a fair statement about Cup drivers participating. At the time, KBM built their own chassis and used Triad Racing Technology engines (so no Cup affiliation at all). KBM's 54 won one race (Kurt Busch at Richmond, 1 more lap and Hamlin wins though) and in the 2012 NASCAR Nationwide Series, the 54 finished 8th in points. It was the only team in the top 10 in points that had no Cup affiliation and none have done it since. The problem is that 13 of the top 15 that year had Cup affiliations (the 44 finished 14th in owner points).

It isn't that the best driver in series' history couldn't win away from a Cup team. The problem is that when he wasn't running equipment or engines from a Cup team, he was typically the only one in the top 10 that wasn't running equipment or engines from a Cup team. The problem isn't that Cup drivers run for Cup teams; the problem is that Cup teams participate at all.

Could you imagine a world where the same ownership limits that prevented more than 4 Cup teams from one organization were implemented to prevent an organization from fielding more than 4 teams total in all 3 national touring series with additional restrictions on technical alliances as well?

All of the top 21 teams in the NXS owner points use Cup teams' engines and/or equipment through either being run by the Cup team, sharing the same owners & having technical alliances (like JRM & HMS), having technical alliances, or buying engines. 21 out of 21.