Welcome to BoneSmart, Timmy. I moved your post and replies into a thread of your own so's not to derail Hearth_Cricket's thread.
Truth is, these numbers are just that - numbers! Don't get too hung up on them. People are walking around now who had their knees put in 15 years ago and even more! Given the developments and improvements in implant design and manufacture and the advance of surgical technique, it's not improbable that knees put in today should be expected to last a LOT longer maybe even 20 years.
Apart from that, I have to say that during my career I've only rarely known people having a second revision let alone a third. No idea why but it's a fact. In any case, 20 years is an awful long time. Why not try concentrating on the here and now instead of something that will happen years from now?