They didn't play in the same era, Witten's rookie year was Sharpe's last year in the NFL. Passing rules changed tremendously over the last 10 years, helping expand tight ends and their role in offenses.
Again, you can't compare Sharpe and Witten, their numbers are irrelevant.
Tight ends with more touchdowns than witten
Tony Gonzalez
Antonio Gates
Shannon Sharpe
Vernon Davis
Dallas Clark
Tight ends that will soon eclipse Witten in touchdowns
Rob Gronkowski
Jimmy Graham
Eventually
Julius Thomas
Maybe even Greg Olsen
If you want to compare him to WRs, he doesn't even make the list in Touchdowns...
Sorry but Sharpe and Witten are from the same era.
Sharpe had 240 of his catches in the 2000s.
He was the original split out TE. He isn't really even a true TE like Witten.
In fact none of your list of guys with more TDs than Witten is.
Those guys are all WRs playing TE.
Guys who will pass Witten remains to be seen, He isn't done and those guys have to stay healthy.
Since the start of the 2013 season Witten and Gronk are actually tied in TD catches.
End of the day it really won't be close.
At some point that will become clear to you.
And yes amongst all positions Witten is on the board tied for 100th in TD catches. He will pass multiple people every TD he scores for the rest of his career --tho some guys will surpass him as well.