I don't think it is the fault of lack of new custom maps. I'd say nowdays maps are MUCH better quality than years ago in all aspects, people improved a lot and it is always quality > quantity.
I don't think so, please give me some examples of the new well-made maps in year 2014 other than Iceborn, I managed to finish it in a week, so the rest of the time I'm just waiting for miracle to happen, waiting for new well-made map and the old maps to update, so even though quality > quantity, I won't repeat the same game for more than 2 times.
Age does effect the popularity, many may dump it (because bored) or don't even try it out (for new folks) just because it is old and there are plenty of newer games around the world that gets more attention, that might be the case why not many new players joins warcraft 3, it is not fresh.
I can't deny Warcraft 3 players populations decreased due to the age, but, players bored of it because it has no update, and the LAN has a 8mb size limit (even though the limit can be removed but many map creators still try to get their maps in range because many players still don't know about it), and new games released such as DotA 2, which is a lot more better than the original DotA, where DotA is players' favourite because it's about battle between players, they love battles (because they always want to be a winner, no battle, no winner) meanwhile DotA update is slow, many features and improvement that can be done are never be done such as mana bar on top of heroes, channeling bar, terrain, special effects of skills and so on. These reasons caused the big difference between DotA and DotA 2 and eventually those players who are rich (quite a number of them) and have fast computers attracted to DotA 2 due to many new features which some of them can even be done in DotA.
Garena is one of the reason too, many players dislike Garena and think Garena always causes connection disruption to their games, around 50% of Warcraft 3 players use Garena, therefore they tend to find a new server - DotA 2 so that they could have smoother gameplay. DotA is the most famous custom map in Warcraft 3, even for now, so eventually most of the players dump Warcraft 3 and went to DotA 2. In a result, Warcraft 3 players decreased. The statements above illustrate about old players.
About the why new players (I assume you refer to new generation) don't play Warcraft 3, few reasons:
1. They prefer those games in smart phone, I-pad, tablets and so on, they spend more time on those games.
2. Everybody plays DotA 2, his brother plays DotA 2, his seniors play DotA 2, even his teacher plays DotA 2, do you think he'll play the original DotA?
Of course it's no longer fresh because many new HD games out there, but they don't have ALL-IN-ONE feature like Warcraft 3 do.
Imagine a new expansion for warcraft 3 by Blizzard, it would reborn.
But this will not going to happen because Blizzard is currently working on Starcraft 2 and World of Warcraft, even though Blizzard finished them, Blizzard would starts a new project, maybe Warcraft 4, or something unexpected by us, maybe Blizzard make a first person shooting game, who knows? So we should just keep playing Warcraft 3 until it's truely died.
So in conclusion, if the production of high quality Warcraft 3 custom maps continues, Warcraft 3 would not die so early.
I'm sorry if I misspeak, please correct me if I did.