It wasn't something I'd really considered before, but I think the point that "you can start more than 1 of them" is a pretty valid one regarding RB/WR even disregarding the positional scarcity it creates.
If you already own a top QB then hitting it big on a good young QB is much less of a boon to your team than it would otherwise be, especially since those guys don't often carry huge trade value. The Rodgers owner in one of my leagues also has Foles, which normally would have been a huge coup. With Rodgers already on his squad though, Foles is only marginally useful for him, and his trade value is far from something that is going to change the dynamic of his team. Compare that to a few years back when I already owned Calvin Johnson and then Demaryius Thomas broke out for me. It didn't mean I had to just leave him sitting on my bench hoping someone else would be interested (though as a WR his value is obviously much higher than Foles' on the trade market), it meant I could just slot him right into the WR2 slot and blow away my opponents at that position most weeks.
In the grand scheme of things it's probably a minor point, but it's something I hadn't really considered before.
I think it's actually a pretty big point, and a reason why I think Gronk/Graham are made more valuable by the fact that most leagues with a flex allow TEs to be flexed (it's unlikely that you'll hit on the next Gronk when you already own the first Gronk, but if you do, at least you can start them both). And it's yet another reason why top QBs are less valuable than top players at another position. But, again, I'm not comparing top QBs to top players at other positions, I'm comparing them to 2nd and 3rd tier players at other positions.
I would argue that there's a flip side to this. The brees owner is a year or two away from drafting youn g qbs in the hopes of hitting the lottery. The manning owner has been drafting qbs for a couple years now even though he's gotten awesome performances from peyton. If they haven't, they're risking being the last man without a chair when the music stops. Its true that breakout qbs don't have much trade value - except to the guy who doesn't have one, and then everyone tries to hold that guy over a barrel for tier two qbs and won't even consider moving their number one. Sure, if you have rodgers and pick up foles, he won't have as much trade value as a guy who has calvin and adds demaryius. And if you want to get max value for foles, you need to trade himsooner rather than later or else you hold all the risk that the eagles pick a new qb of the future. So it takes a perfect storm to get that true stud qb and actually find a trade partner who will give you full trade value for him during the window that you're willing to make the deal. And that's a very valid point.
But if you have rodgers, what are you doing drafting foles? Part of the value of a young stud qb in his prime is that he allows you to draft deeper at the higher priority positions for the next ten years while the other owners are mining for qb talent. If you never hit a draft pick on foles, you also never waste a fairly early pick on gabbert or ponder or freeman. That is a large advantage when you hit on a guy like demarco murray, or zac stacy, or aaron hernandez, or some of the other guys who've slid into the late first/second round territory the last several years and become uber studs.
I think the only one qb problem is much larger in redrafts, where you can pick up the 25th ranked qb off waivers when he has a big week. In most dynasty leagues, its rare for any qb to show up on waivers - maybe foles made it to waivers, but he didn't in my 14 team league. Before that I think vick was the last relevant waiver qb. The low opportunity cost of qb mining in smaller dynasties or redrafts devalues qbs, because it keeps early qb drafters from being able to get lucky in as many ways as late qb drafters.