I don't believe the problem is Cup drivers entering in Xfin races. I believe the problem is that the Cup teams run the show, and the non-Cup teams can no longer compete. Like not even close. I don't know what factors have led to such disparity, but I think it's killing interest in the series. All the major Cup teams are now represented in Xfin, and the gap between those Cup teams and the other guys has reached an all-time extreme.
Yes, Kyle Busch wins half of the races he enters, and leads a million laps. But 20 years ago, was it really that much different? Mark Martin raced a dozen BGN races a year in a Roush car and won quite often. The Labontes did it, Earnhardt and Gant and Irvan did it, Jeff Burton did it, Dale Jarrett did it, hell even Jimmy Spencer and Joe Nemechek did it. Even back in the 80s, Grand National races had several Cup drivers entered on a regular basis.
The difference back then was that the BGN teams could actually compete with the Cup teams/drivers. Guys like Steve Grissom and Randy Lajoie and Chad Little could run toe to toe with anyone. Not anymore they can't. The good BGN teams have steadily declined over the years to the point where Turner Motorsports was the only team that could run with the big boys, and that's been a few years ago now.