Given that 13th Age is releasing books and 4e is not, I think it's going to appeal to a lot of them.
13th Age will also appeal to 4e players who:
- Appreciate the elegance of the 4e ruleset but are getting tired of hauling out the battle mat for every single combat.
- Enjoy tactical combat but are ready for a system where fights are a little quicker.
- Feel that WotC hadn't completely explored the 4e design space before they killed it.
Intentionally or not, the 13th Age rules are going to feel really familiar to 4e players in all the right ways. The game has ongoing support, unlike 4e. It simplifies combat without entirely throwing away the tactical aspect, which should appeal to a lot of 4e veterans. And it's eminently moddable, which should appeal to their DMs.
If you're a 4e player who is disappointed by the direction 5e appears to be taking, 13th Age is looking really good right now.
Will it be around in a few years? It's an indy game, so it's hard to say. But the core book has been so wildly successful (by indy standards) that the publisher is rushing out more books for it (the Bestiary and an 'unearthed arcana'-style expansion called 13 True Ways should appear within the next 3-4 months). If ever an indy game had a shot at long-term success, this is it.