Sure, the past
can help predict future performance
but the problem is that a WR's performance is largely tied to the system he's in. So basing future predictions off of Crabtree's SF days is stupid. He's not in SF anymore. He's got two years with Derek Carr under his belt and he'll have three more. Ignore SF, just look at the relevant years. As for DT, I'm sorry but the days of 4500 yards and 45 TDs passing in Denver are gone. Ignore those years because they aren't relevant anymore. Peyton isn't coming back. Look at the last two years. That's about what's he'll have going forward. Denver's aggregate QB last year was 12th in attempts and 14th in passing yards, so it's not like Old Peyton + Oz = absolutely horrible as you say. That many yards and attempts still produces WR points in a PPR format. That range of QB performance is probably what DT will be dealing with for the next few years, so 2015 and 2016 should be pretty good predictors for him. (so far this season, DEN QB = 18th in yards and 16th in attempts)
I don't know if you follow Matt Harmon's reception perception, but I'm a big fan. He broke down DT's film and I could be mistaken but my recollection was that he concluded DT was a good, but limited WR. So I disagree that he's elite. Ok, I took the time to look it up.
Here's the article. I won't quote all of it, but here's a snippet:
Reception Perception illuminated what diligent tape watchers saw with their eyes. Demaryius Thomas, for all his prolific stats, is a glorified role player. One of the NFL’s best receivers, but inherently limited in what he brings to the table. Thomas posted an above average SRVC score on only four routes—the screen, slant, post and nine. You’ll also notice that he accumulated almost all of his production or PTS on those same patterns. It’s striking to see one the NFL’s most prolific statistical producers grade out so limited.
Regardless of our labels for these guys, my questions that you didn't answer are still worth considering, so I'll ask them again:
How confident are you that DT will outscore Crabtree in the next three years? Who do you think will be more productive at age 33+?