I think it's a bit early to be saying "only way he doesn't go top 15". It's possible, but 15 is a very tough threshold to break through. Consider some of the defensive competition he'd be up against:
DT Lotulelei
DT Hankins*
OLB Jones*
DE Montgomery*
DE/OLB Mingo*
DE Werner*
DE Moore*
DT Richardson*
ILB Te'o
CB Milliner/Banks
Ansah is a phenomenal athlete and he's progressing very quickly, but top 15 is a pretty high pick to spend on an athlete. Look at how Kansas City's done with Dontari Poe, a phenomenal athlete with tremendous upside but who was probably better suited to be a late 1st round pick based on his technical development compared to his athletic ability.
It's possible that Ansah will go top 15, but I don't think it's a foregone conclusion at this point, but any means. And if he does, it means that one or more of the above guys will get pushed down.
Here's why I think he will go top 10
1. You mention Dontari Poe, don't forget Stephon Gilmore. Had late 1st, 2nd round ability but had ideal length and athleticism for his position. Went top 10. And of course, we musn't forget Michael Brockers, offered little in the way of pass rush but had ideal size. So that's three players last year alone.
2. Your point on all the competition is well taken, but for most of the pass-rushers, their 'stock' is mostly falling. Mingo and Werner haven't put up great numbers, Montgomery was being talked of as top 5, now top 20, Jarvis Jones has neck issues which may raise red flags and isn't a perfect physical specimen for his projected position (which in itself is a bit of a problem, where does he play?)
3. Check out the play at 0:10 in the below video. When you can show that level of fluidity at that size and length, you are bordering on being unique in the NFL.
Ezekiel Ansah vs Idaho 2012 - YouTube
4. Bearing in mind how far he's come in just over one year, NFL DL coaches will be absolutely salivating at the prospect of getting him on their team.
5. I don't know how many batted balls he has but he can't be that far behind JJ Watt, and Watts' have been real difference makers. Also, long arms help force fumbles with players like Ansah or Chandler Jones reaching over blockers to swat the ball out of the QB's hands.
6. Considering all the attention he's had to date from NFL teams and the fact that he'll likely interview well, some team will fall in love with him.
7. He's not scheme specific, could be equally comfortable in a 3-4 or 4-3.
8. 4.79 40
34.5" vert
6.83 shuttle.
Those are Melvin Ingram's combine numbers. Ansah is being talked of as 4.6 40 and 39" vert. Ingram had 31" arms, Ansah looks more like he has Chandler Jones arms.
9. This time last year, Bruce Irvin was a 3rd rounder, Mclellin a 4th, Chandler Jones a 2-3rd and Melvin Ingram a 2nd. Ansah's stock is already on the up and up, I just don't see the momentum slowing when all that's left for him is Senior Bowl, combine and interviews (presuming he lives up to expectations in those).
10. 7 of the players you listed are juniors. 17 of Rob Rang's top 32 are juniors. If he's at 31 now, almost by default he'll move up once declarations are made/not made.
Anyways, thought I'd just shoot from the top of my head. To be honest, I'm starting to wonder whether top 15 might be too conservative. Rather than worrying about whether we should be drafting him, perhaps we should start worrying about the Jets getting him.
Edit: Of course it isn't a foregone conclusion, I'm only making a prediction based upon my very limited experience with the draft. Manxman being wrong would not be the shock of the century
