They weren't constrained nearly as much as IP constrains attractions, which is the only thing that matters in this discussion. Non-IP rides have a much stronger track record at Disney than IP rides.
Again, it's not that it's impossible to make a great IP attraction. It's just much harder than making a non-IP attraction great, because there's far less freedom to be creative.
I want great attractions. We are far more likely to get great attractions when IP isn't mandated -- all you have to do is look at EPCOT to see that, but there are plenty of other non-EPCOT examples. I'm not opposed to IP attractions, but requiring it really limits what designers can do and will almost inevitably lead to lesser attractions overall. Unfortunately, that's not likely to change anytime soon.