1 - WM30 was 100% Lesnar's fault. His sloppiness caused Undertaker's injury. There's always the possibility it could happen again, but if Ric Flair can wrestle well into his 50's without suffering a serious injury, so can Undertaker.
2 - Yes, it gets far MORE repetitive with full-time performers. I'll take another Lesnar/Undertaker match over yet another Ambrose/Rollins match any day of the week.
3 - I already answered this in my other post. But the simple answer remains that the VAST majority of fans will cheer Undertaker over Brock Lesnar no matter what.
4 - "Logic" in terms of who would win a real fight is completely and totally irrelevant in WWE. Going by that kind of "logic", a magical middle-aged zombie wearing eyeliner never would have had 21 straight victories at WrestleMania; Rey Mysterio never would have won a match, ever, against any competitor; and Mark Henry would be undefeated, because his bear hug would result in instantaneous submissions from any opponent.