People threw away their system instead of digging deeper into it
I don't think this is really true for most UFC fighters.
Fighters who came in with a foundation in wrestling didn't abandon wrestling. They learned how to adjust their wrestling for an environment where they could get punched, kicked, and submitted. Along the way, they generally developed some good skills for punching, kicking, choking, and arm-locking along with their wrestling.
Fighters who came in with a foundation in Karate didn't abandon Karate. They learned how to adjust their Karate for an environment where they could get taken down and submitted. Along the way, they generally developed some good skills in wrestling and submissions.
Fighters who came in with a foundation in BJJ initially did very well because they had a tradition of training against wrestlers, karateka, boxers, etc. But a large part of that early success was due to the fact that their methods were a mystery to their opponents. Once all the fighters in MMA were wise to their tricks, then they had to up their game by getting better at striking, wrestling, and even the foundations of BJJ.
What we did see after the first few years of MMA was that most of the would-be fighters who insisted that they didn't need to cross-train because their preferred art held all the answers just stopped showing up to compete, because they couldn't even get do weel at the amateur level, let alone the elite professional ranks. Nowadays we see an increasing number of fighters who come in without a foundation in a single art and just start training in "MMA". i.e. a given gym's synthesis of the essential techniques and training methods for MMA competition. But there are also still plenty of fighters who have a strong foundation in one or two arts and continue using those during their fight careers.
I'm not saying that there are no fighters who started out with a strong base in Wing Chun or TKD or whatever and then completely threw it out in order to go study BJJ and subsequently had a significant MMA fight career. I'm just saying that I can't think of any offhand.