"almost no outside pressure to adapt and evolve"
THAT...is one HUGE assumption! And spanning 360,000,000 years! Am I to just take your word on that? LOL
Yes.
It's a well adapted, parasite that feeds on blood. As long as there are other creatures with blood (and no hands with which to remove it), the lamprey will eat. Any population of lampreys that were acted on by significant forces may well have evolved into something else but the basic design of the lamprey and it's niche is well protected. So long as creatures with blood live in the sea.