People don't play DotA because it has skins. People buy skins because they play DotA.
No significant number of players will rush back to Starcraft because the balance has been tweaked (again) or because they can now get an Abathur voice pack. The Starcraft community seems to be in denial about the fact that the problem with the game is the game.
Brood War was primarily a single player game. Secondarily, it was a game where people who had no idea how to play RTSs well logged on and played custom games and 4-8 player money maps. A long, long way down that line, it was a game where people learned to get good at RTS-playing and competed with each other 1v1.
Starcraft 2 exists in a different universe.
On the custom map front, it competes with thousands of cheap or free indie and mobile games that offer better experiences because they're not jerry-rigged on top of some other game's engine.
On the multiplayer front, it competes with a new generation of online multiplayer games - DotA and its descendents - that appeal to the same audience, and were eating the traditional RTS player base alive even back when it was just a WC3 custom game.
And on the "getting good at it and competing" front, the new generation of Starcraft players are learning that getting good at an RTS is a very niche taste. Getting good at Starcraft means a lot of rote training in multitasking, and until you get over that major hurdle, the feeling of playing the game is of being helplessly overwhelmed by all the things you can't keep up with. To quote a great comment from /r/games I read a while back:
Casual players usually think that RTS means they should be able to use any strategy they want and have a chance to win. This usually means turtling, but not always. When their favored strategy turns out to be completely ineffective, they complain about the game being bad instead of trying to find an effective strategy (which in theory is the entire point of a strategy game). As long as they never touch competitive multiplayer they don't have to learn that their strategies suck so they like the game.
…When you're playing casually you can play very slowly and still beat the AI or your casual friends. But when played competitively every RTS game requires the player to be constantly doing something, never idle. This is inherent in the nature of real time games.
By pushing casual players towards competitive multiplayer, SC2 made them realize these things. This quickly killed many casual players interest in the genre. Of course, they usually attribute these problems to SC2 alone, and complain that every game since has just been copying SC2, but these "problems" have existed in every RTS game, they just never experienced them competitively.