Guys like Royce, Shamrock, Severn & many of those old era dudes would've been good in this new era in their primes.
But they all would've lost to someone, that's just the nature of MMA, it's **** near next to impossible to stay undefeated your whole career. The striking & grappling is just too advanced in this era.
Royce is arguably one of the best BJJ practioners of all time, but Eddie Bravo proved with the Rubber Guard that even the Gracie lineage isn't invincible, when he basically beat Royler twice.
My favorite fighter of All-time never competed in MMA, I wish he would have, but he was arguably the greatest non-Thai Muay Thai fighter ever, Dany Bill from Cameroon. He (& Bruce Lee lol) is the sole reason why I got into martial arts.