It's true that the league has changed, but I would think that the league changes would only further favor the wide receivers.
I think the opposite. Rerun your analysis, but this time, instead of "RBs picked between 49 and 59", say "the first running back drafted". The market hasn't just changed a little. The top free agent running backs didn't get paid squat. The first running back didn't go until the late second. That may be an indication of the talent available, or it may be an indication of the market. The flaw in your analysis is that using draft slot heavily biases the analysis with the assumption that it's an indication of the talent available.
Moreover, just because Watkins is an elite WR prospect, who cares? If you polled their current owners, you'd find out that there are dozens of top ten dynasty receivers. Watkins might belong in that pack - he might even be at the head of that pack - but he's in a crappy situation. Name the cold weather receivers in the modern era who put up elite fantasy seasons without a HoF quarterback. It's a pretty short list. AJ Green counts, I suppose. So does Braylon Edwards. I'm not sure if that's good or bad news for Watkins. But the point remains that looking ONLY at the draft position of a wide receiver to determine future success without considering their QB or weather situations is flawed.
And while I fully understand that teams can change quickly in the NFL, my concern is that Manuel is good enough to keep that job, but not good enough to make Watkins a contender as the #1 receiver in FF. And that's my real issue. He might flirt with top ten, but lots of guys can lay claim to that over the next few years. I don't know if that's good enough to command the 1.1 pick.
People are quick to blame the devaluation of RBs on "the market". Personally, I think it's just more a matter that RBs entering the league recently have sucked, comparatively speaking. The only phenomenal prospect in recent years, Trent Richardson, still commanded a top-10 pick *AND* another first a year later.
Three years ago, if you weighted RBs by production, the average back in the NFL was about 2.5 years younger than he is today. You know what that tells me? The guys who have entered the league in the last three years have been spectacularly unproductive relative to the guys who entered the league more than 3 years ago. Are RBs less valuable today? Maybe a little, but more than that, we've just had a dearth of really quality prospects entering the league. Just witness what happened after the 2008 NFL draft and arguably the most talented RB crop the league has ever seen.
Anyway, I think emphasizing situation SO MUCH over talent in the rookie draft is a losing recipe. Situation changes, especially over the timescale we're looking at when it comes to rookie development. Who knows what the league will look like 3 years from now, when these guys are becoming fantasy cornerstones for the teams that drafted them.
As for your "how many receivers have elite seasons in cold locations without HoF quarterbacks", well, that's necessarily going to be a pretty small number because you added so many qualifiers to the list. I mean, how many cold weather teams are there? Green Bay, Chicago, the Jets, the Giants, the Pats, Denver, the Eagles, the Steelers, and the Bills are the ones that come to mind. Immediately remove everyone from the Giants, Packers, Pats, Steelers, and recent Broncos teams because of the "HoF QB" requirement, and you're looking at maybe 15% of the receivers in the league that would qualify. But, to answer it anyway: Terrell Owens, Braylon Edwards, Brandon Marshall, Alshon Jeffery, A.J. Green, Santana Moss, Chad Ochocinco, DeSean Jackson, T.J. Houshmandzadeh, Percy Harvin, Lee Evans, Rod Smith, Eddie McCaffrey, Javon Walker, Amani Toomer, probably some others I'm overlooking. And the entire question creates a false dilemma- even if you like Sammy Watkins, the default solution is not necessarily "draft Sankey, instead". There are a lot of other choices at #1 that don't involve reaching on a late-2nd-round RB.