If that were true, wouldn't boxers remain champs for incredibly long amounts of time with the giant gloves they have?
I think the reason it feels like MMA fighters stay champs for so long is because they only fight a couple times a year. They have far fewer fights over the course of their careers than kickboxers or boxers.
I think the sport has also progressed on every level - striking, wrestling and grappling - from where it was in the early days, as well. The training is more professional and the level of technique is higher.