Mods come and go, mods also change (e.g. extradimensional barrels no longer exist in 1.6.2), the only way we will know if Unleashed is going to last is to look at the mod landscape in the future and see if there are sufficient mods and sufficient compatibility (which includes Forge changes) to make worlds work when upgrading to the new version of Minecraft. Clearly there was not sufficient compatibility to allow Ultimate to survive the transition to 1.5.2. It was not planned nor deliberate, and neither will the inevitable retirement (at some point in the future, maybe 1.6, maybe 1.7, maybe later) of Unleashed be. But just as you can continue to download and run Ultimate on 1.4.7, you will always be able to download and run Unleashed on 1.5.2 (until, that is, FTB Inc. goes bust and are bought out by Tekkit Megacorp).