I think if you look at the how first round picks have historically panned out, you'd find that simply isn't the case.
I understand your point - but to me, a failed pick is a failed pick, even if the league average is that only 50% of Day 1* picks stick in this league, which I think is what it is.
*The old Day 1.
Maroney was also the 21st spot in the draft - here are some other guys drafted at 21 in recent memory - Alex Mack, Sam Baker, Reggie Nelson, LoMo, Matt Jones, Vince Wilfork, Jeff Faine, Dan Graham, Nate Clements. So the 21st pick provided 2 studs (Vince, Clements); a bunch of career starters [or guys headed on that path] (Faine, Graham, Mack, Baker), a headcase (Jones), and two guys who are frustrating their fanbase (Maroney, Nelson).
So looking at the #21 pick over the last decade, Maroney is probably in the bottom tier, success-wise.
Then you can look at it in terms of the Patriots...Maroney is arguably the worst pick this team has made in the 1st round this decade. Obviously, the only other pick who has disappointed is Watson. Like Maroney, he teased us with flashes of athleticism, but ultimately gave inconsistent production. But, with Watson, he was the last pick in the first round, and did give us 6 years of service, and contributed well as a blocker in his last few years.
Don't get me wrong - I like Maroney. He's a solid NFL running back with some nagging problems (inconsistency, injury). When he's healthy, up to this point in his career, his metrics show he's on the verge of becoming a top tier RB. Unfortunately, his anti-clutch play & his injury woes have derailed that from happening. What it comes down to is that we likely could have grabbed a player in the mid-rounds who would have produced in a similar manner as Maroney. Philosophically, I think RB isn't worth it in round 1 unless its a sure thing.