I disagree with this. If growing knowledge of sweeps killed sweeps in MMA as an effective technique, then why didn't growing knowledge of submissions kill the submission game in MMA?
I'd argue that the issue is simply a matter of preference. Most people like to submit from their back rather than sweep. At our academy we even do "sweep only" specific training to force guys to spend more time learning how to effectively set up sweeps instead of just throwing up submission attempt after submission attempt.
Neither submitting nor sweeping is "easy." But I think it is incorrect to suggest that one is inherently more difficult than the other in MMA.