Being the best indicator doesn't mean it is still not a dicey indicator. The stats confirm it in my mind. But I have had this argument numerous times in this forum over the years with people who swear by the draft pedigree/combine numbers model, which I consider junk science. But that is just my opinion and we will have to agree to disagree on this.
Anyway, check out the actual numbers and look at drafts after 5 years. About 50% of first rounder pick succeed, at best, and there is a significant drop off percentage wise in each round after that. So, for the first round that is a coin flip. And Bloom had a posting in a thread some years back (which I can't find) pointing out that the professionals whose only job it its to scout the best players have a terrible batting average.
From an April PFT piece by Mike Florio:
http://profootballtalk.nbcsports.com/2016/04/22/the-first-round-of-the-draft-remains-a-total-crapshoot/
				
					
						The First Round of the draft remains a total crapshoot
					
					During Friday’s 
PFT Live on NBC Sports Radio, I asked Falcons G.M. Thomas Dimitroff whether the success rate at picking quarterbacks at the top of the draft. 
Dimitroff offered a more general observation that underscores the uncertainty of the first round, at any position.
“That’s always an interesting discussion and we talk about it all the time,” Dimitroff said. “What’s funny is we talk about that with quarterbacks but then when we start looking at positions that we’re interested in. We can look at interior D-lineman over the years or [pass] rushers or whoever they may be, and we all want to come up with this stat that says, ‘Wow, this is an easy pick.’ It’s not an easy pick in the first round.”
It’s not easy because it remains, at best, a flip-of-the-coin proposition.
“According to our most recent statistics that we drew on the first round, it’s less than 60 percent of those players that are starting,” Dimitroff said. “I think it may have come in at 56 percent. So point being it’s not an exact science, we know that. There are so many other things that are involved in it. The first step is finding out whether that player has the adept skills on the field, of course. Many other areas that we’re looking into to make sure they’re fits in the organization. [Do] they have the mental capacity, they have the character capacity, and the team element that a team is looking for? Again, you’d better have a plan for the guys who are a little bit wayward in their approach. That’s alway been a big discussion point as well.”