it's an interesting move, but one that I think will turn out to be less significant that many assume unless Amazon really expands the program to cover a majority of the wines out there.
1) the common, everyday wines are still ones that people will want to pickup at the market as they shop for food. It's the "grab a bottle for dinner tonight" impulse and no online program will satisfy that.
2) Wine retailers will retain their local customers for the wines that they carry in common with Amazon for much the same reason. This will be less true if the retailer's customer base is spread out over a large area as the convenience of shipping is larger then.
3) Online retailers have little to fear either in the short term IF they sell mostly fine and allocated wine AND if they also sell older wines.
Online buyers are driven by either knowing that retailer and their reputation or by searches on Winesearcher.com, winezap.com etc. The main worry I'd have if I sold wine online would be that Amazon would overlap my stock and compete with me on price. That WILL be a threat to online sales if my online sales. To the degree that my wines are things Amazon can't get (older wines from cellars, allocted wines) the threat is less.
Service will be a differentiating factor here too - if I buy a case of high end wine in July but have it held for shipment until the weather has cooled in November that's important to me. Is Amazon able to do that? Or will we see a ton of heat damaged wine out there because they'll ship that case of wine in 105F weather?
Also I wonder about the uptake of this by wineries. I think you're spot on as to which will be interested, but if I were a winery concerned about MY rep, I'd want to know far more about storage of my wine and shipment of it during the May - October timeframe. No, this isn't totally under my control when I sell to a distributor either, but somehow Amazon feels different as if more of any negatives will be borne by the winery.