Yeah, but even Jeet Kune Do has already died out almost entirely in favor of a combination of (Muay Thai, BJJ, and either Greco-Roman wrestling or Judo) aka MMA. Maybe throw in some Krav Maga for defense against common street attacks and you're set.

Realistically, hybrid martial arts have been around for a long time. When I was a kid (long before the MMA hype train), I did Karate, but it also involved learning ~30 judo throws, a lot of jujitsu, as well as a wide variety of weapons (bo, nunchucku, kama, eiku, sai, tonfa) as all of the instructors also were black belts in some Okinawan weapons art. It was a lot of fun and better than any martial arts training I had previously (made the years I spent on Taekwondo feel embarrassing), but still nearly useless against a mediocre Muay Thai and BJJ fighter.

Well, clearly. He developed JKD decades ago before MMA was ever a thing as we know it now. That's not a slight on JKD, and if he were still around, whatever art he'd be practicing now would be closer to modern MMA than JKD. He was entirely against keeping up techniques/ideas out of tradition. His philosophy was to find what works. If it turns out something stops working, replace it. He was never so married to any idea of a "perfect" martial art or anything. He was always experimenting and trying to evolve away from traditional MA which was entirely stick in tradition and unable to progress/improve.

While not modern MMA the Gracie brothers were doing their vale tudo fights before Bruce Lee was born. Though obviously they were mostly using the techniques that today is called BJJ the fights themselves had almost no rules. This also meant no rounds or stand ups so if a BJJ fighter managed to pull a striker to ground the fights is effectively over.

Problem back then was information. Pre internet and stuff happening really far away from each other (Brazil and Hong Kong/USA).

UFC and the internet really helped to spread information on what actually works to a much wider audience (and from there to a group of practitioners) and thus sped up the development of the techniques a lot.

UFC also made it possible to make a living from actually fighting instead of teaching others which is/was the “traditional” way of making money from martial arts which allows the fighters to fully concentrate on their own skills.

"Bruce Lee's movie fighting style" then, which has little to do with his proper fight training, like Jean Claude van Damme's fight choreography has little to do with his fighting background.

Apart from that, I do take issue with the for some reason increasingly popular notion of Bruce Lee's fighthing being some sort of proto-MMA. MMA is not just taking something from various sources and calling it "<first name>-fu" or "<first name> tang doo". The essence of (old school) MMA/Vale Tudo is about finding out what works and what doesn't through literal trial by combat, it's not about the mix and match aspect.

So, exactly what Bruce Lee did. Read his books. He talks about how he was disappointed with traditional martial arts and how he was always interested in finding the most effective techniques and ideas.

I honestly don't understand why you're so eager to criticize what he did when you don't seem to know much about what he actually did and what his thoughts were about martial arts.

Please show me the full contact fights Bruce Lee was in.

That essence is what Bruce Lee espoused as well, from what I've read. Apparently that's what got him into trouble with the traditional "elder" practitioners.