I disagree. I think those fighters were talented. I think generally speaking they would be just as far ahead of the competition now, as they were back then.
Silva, Aldo and GSP weren’t so good exclusively because they started training at a younger age. They all started training around the same age as their contemporaries, just like these fighters now.
And even now a days, there are top fighters who started training late and demolish lifelong guys… (Volkanovski, Pereira, etc).
It’s all about the same; they’re just not as special and talented as they used to be.