Every team relies on their WR blocking well. It's part of the game. But it's pretty rare to have two great blocking WRs like Rod/Eddie on the same team.
Shanahan and Kubiak know Lelie's limitations and capabilities in the blocking game and have designed plays to account for that for the past two years. Yes, our running game could be a lot better with Lelie blocking like Eddie Mac but that's realistically that's not going to happen. Good O-coordinators find a way to maximize their plays and hide weakenesses like poor blocking.
The Broncos running game is unlike any other in the NFL (save Atlanta's now) as we all know and they depend on their WR's for downfield blocking more than other teams. It's no coincidence TD goes for 2000+ with Eddie Mac and Rod in there. I agree that all OC's can design around flaws but that's not the point. The point is that Lelie has had some limitations due to his lack of physical strength, a fact you just admitted with the comment about him getting pushed around by CB's in the short zones. However, it appears that's about to change if what we're hearing about his weight room gains is true.
As for Porter, he is a more complete player at this point IMO, but the key phrase here is "at this point". I think Lelie has a considerably higher upside. He has elite speed, something Porter does not have. Porter also has a 2 year edge on Lelie in terms of experience 5-3, and both players took till their 3rd year to have breakout years. Porter has not gotten significantly better since his 3rd season, meaning he's probably peaked, so what he is now is what he'll continue to be till he eventually declines. Lelie on the other hand has considerable room for improvement and if it's true he's getting considerably stronger, that equates to a much more valuable receiver in the short/medium zones. Lelie as a medium range threat will force CB's to play him closer instead of backing way off. That's where I think we'll see his game explode. He's capable of 12-15 TD's a year if he's developing a short/medium range game to go with the deep stuff. Also, I think the 21 ypc figure is startling given the fact that Plummer does not throw a great deep ball. His arm is average and throwing on the run doesn't lend itself well to deep passes either. Porter's ypc jumped to 15+ last year, probably reflected by the fact he has Collins instead of Gannon throwing it. I don't see him extending it much beyond that. Lelie could develop an underneath game and probably still avg 18 ypc IMO. They're different types of receivers but I think based on age and upside, you gotta take Lelie. 6'3", 205 with 4.25 speed doesn't grow on trees. Lelie has the capability of developing the underneath game Porter already has. Porter does not have the ability to develop the deep game Lelie has. If Lelie can post 50+ catches and 1100 yards without legitimate NFL strength numbers and hence no short/medium game, it stands to reason a major jump in that department equates to significantly higher production. Avoiding injury, I see no reason with the aging of Rod Smith, that Lelie can't become the #1 guy this year or next. 80-100 catches and 1500 yards/12-15 TD's are obtainable goals for him. Porter will never approach that.