The question asks whether or not traditional martial arts practice still holds value in the era of MMA.
From an MMA perspective, TMA still holds some value, as evidenced by MMA people who are still incorporating aspects of traditional martial arts in their MMA training to various degrees of success. They would typically only take a small idea from TMA and then adapt it to MMA. And they can only do that successfully after a strong core foundation of MMA technique has been mastered.
Lyoto Machida was a good example. He came from a karate background, adapting ideas from karate into his MMA game. For a while, Machida was the best. Nobody could beat him. He didn't take many ideas from karate, but what he did take proved to be very useful.
Of course, it didn't last forever. Machida's opponents eventually figured out how to deal with his karate style, and he started losing. But those were elite competitors who seriously study their opponent. To you and me, we might still succumb to that karate flavored MMA, even if we train in MMA for 10 years. It's still useful. And honestly, there is no "unbeatable" technique, period. Sooner or later, someone will figure out counters for everything.
So I take a long view on this. In the 1990's, MMA wasn't really well defined. Nowadays, MMA has a fairly universal core foundation of skills, and everyone starts with that, with a lot of room for individuality of course. Getting good at that means you're able to add more to your game, taking ideas from TMA. It took a decade before people were ready to do that, because that core foundation wasn't really discovered until fairly recently.
Now that people are able to quickly come up to speed on MMA skills, they have the ability to explore concepts from TMA. Most will not succeed at incorporating TMA techniques into their MMA game, especially not at first. It takes a particular personality to stick to it and figure out what they could do to adapt it to MMA. Some, like Lyoto Machida, will figure it out. But they're rare individuals. Which is why most TMA techniques don't show up in MMA competition.
Is it worth training in a TMA for decades just to be able to extract a tiny fraction of it to use in MMA? That really depends on the individual. I love Chen style Taiji, for example, yet I know I would never get good enough to be able to use it alone as a fighting style. But there are skills in Taiji that are kind of nice to have, which take 10+ years to really get good at. These nice-to-have techniques may be your secret weapon in MMA that gives you just enough of an advantage that you can use it to stand out in a crowd. You can't just look at TMA videos and decide you're going to take something out and use it. It really does take directly training in these TMA styles to get good at it first. And then you have to do the hard work of adapting it to MMA rules.
At some point, also, you're going to get old. Everyone does. And injuries you took long ago will mean you can't do some of the things you used to be able to do. Maybe your knees will get arthritis, preventing you from doing most things involving kicks or even evading quickly. MMA requires athleticism. So at some point, your body will tell you it's over, you can't train in MMA anymore. Yet, you still want to do something martial. A lot of people seek out TMA training when they realize that. Taiji is a great example of one that many older martial artists gravitate towards. Classical Japanese Jujitsu is another one. These are styles that either don't have or don't emphasize competition and athleticism. Yet, they have techniques that might interest you on an intellectual level while keeping your body active and fit.
You could imagine a world with no TMA whatsoever, just MMA. In that world, it would take a very long time for new ideas to pop up. MMA trains people to think a certain way, and so they're just never going to conceive of ideas that would have been found in TMA. So in a world without TMA, I'm afraid a lot of ideas simply wouldn't arise.
I don't think TMA will eventually die out and disappear. There will always be people who don't want to do MMA, for whatever reason. There will always be people who think TMA is "good enough" for practical self-defense. And there will be folks who still insist TMA is better than MMA in certain ways (never mind if they're correct or not). But the existence of TMA will only help MMA become better over time.
Hope that helps.