Very much this. Teams who don't want to spend will always find ways not to spend. The Coyotes routinely found players who were effectively retired, who the other team would still pay, but would take on their cap hit to get to the floor. We should expect the Pirates, Reds, and others who tend to refuse to spend to find whatever loophole they can.
I'm in favor of a cap/floor situation only if it's based on % of revenue and that % is around the NFL level (like it can't be so heavily skewed to the owners that it makes it moot). However for that to work, MLB would have to give up their anti-trust exemption and open the books in a legitimate way. Which, let's be real...it ain't happening.
Beyond that, MLB and other leagues run about the same levels of parity. Right now the Dodgers are a bit of a runaway train, but the NFL had the Patriots hegemony with a cap/floor. And yes, teams like the Pirates feel like they never try, but the Browns have been destitute forever.