All-in-all, I think Hasbro has done a decent job with the Axis & Axis collectible miniatures, especially considering that it doesn't include any spellcasters or spaceships.
As a primarily history-oriented wargamer, I was excited when Hasbro introduced AAM. I figured it probably wouldn't last long, but it would be a chance to introduce more players to historical wargaming, as it would be on the shelves right there with the D&D and Magic: The Gathering stuff.
Much to my surprise, the land miniatures game turned out to have enough staying power for eight sets, and the third set for naval miniatures line is imminent, with a commitment from Hasbro for a fourth set.
That's not to say that there haven't been some rough spots, particularly for the land game. There have been some quality control issues and a controversial scale change. The naval game's path as been smoother and of the two, I think it's really the better game. The naval theme seems to be a better match for the collectible format and the rules seem a bit tighter for competitive play.
Collectible games, other than Magic: TG have a very high failure rate, so I think it's interesting that AAM and WAS have hung in there.
I think there are three reasons for its longevity.
1) New releases have been well-paced, perhaps even a little slow. This gives people time to accumulate models on a budget and builds anticipation for the next release. Leave them wanting more.
2) Collectors are a higher proportion of the market than usual, so there's a good balance between competitive players and collectors. The inherent collectibility of the models provides a bigger customer base than purely competitive play would, making sales steadier and less faddish.
3) Suitability for other uses. The models in both WAS and AAM are easily used for other game systems. Indeed, the controversial scale change in AAM was made to better align the models with hobby standards. While the WAS models are a unique scale, it's a good scale (small enough for player convenience and lower cost while big enough to show good detail even on small ships such as PT boats, sub chasers and submarines). And there are now enough models to make the line pretty self-contained. There are already enough ships that the line compares well to lines offered by other manufacturers and if a few more sets appear the problem will soon be finding enough suitable ships. (Already cropping up with the German navy).
The collectible format has its irritating aspects, which are well-known and don't need repeating, but if you're going to do a collectible game, I think Hasbro hit the mark with Axis & Allies.