Would a 15% alcohol wine lose its alcohol over time? Say in three years it'll be 13% instead of 15...?!
Answers: The bottle will remain uncorked, of course, just in case you wonder...
Uncorked means no liquids escape and no gases get in. Thus, the 15% will remain 15%, since ethanol does not decompose into vinegar and water (your liver turns it into this, however).
Some bottles of wine have faulty cork, and will turn into vinegar. It's hard to say if its "corked" without opening, though.
Uncorked? I think most of it (alcohol AND water) would evaporate if you left it like that. If you left it unopened, though, there shouldn't be a loss of strength (it might start to taste kind of funky after awhile, though).
Actually if its red wine stored in a cold dark place it gets more prominent in taste. White wine however is not supposed to sit more that 3 days after being bought, the fresher it is the better it is
No reason to think alcohol would escape or degrade in that period of time.
If anything, over time, more sugar may actually be converted to alcohol.
First of all, there are very few wines that I am aware of that have a 15% alcohol content. If I am not mistaken, that would be Zinfandel in an extremely ripe harvest year. See the Zap Festival link for a list of good vineyards.
As far as losing Alcohol over time, if the bottle is on its side and cellar chilled or kept cool, not likely unless it was damaged in shipping. After say 12-15 years, the nature of the wine starts to change as a slight oxidation does occur in the bottle, however, the Tannins from the grapes minimize oxidation if it is a good wine.
I hope this way helpful to you!