Are there any systems in place to keep low-level crafting useful?

Pretty much what George said. And afaik SWG had an even deeper system and Ashes is going for a crafting system like that. And Steven has played L2 so at the very least he could rely on that.

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I think that could be counterbalanced with the world manager. Say those lowbie swords give a particular kind of material on deconstruction (not present in the crafting stage). That material is only useful in higher lvl crafting, so lowbies wouldn't want to deconstruct swords.

That material is also acquirable through some other means that can be controlled by the world manager (quests, mob drops, etc). The world manager looks at how many lowbie swords have been crafted and deconstructed and then adjusts the highbie acquisition values accordingly. If the majority of lowbie swords have been deconstructed - the highbie methods get easier, which brings down the price of the material and also makes the swords method inconvenient. If majority of swords do not get deconstructed (this could probably get tied to some arbitrary timer too) - the highbie methods run dry.

Obviously this would need to be tested and refined, but I do think that you could control this through the planned systems, while also basing it on player actions.

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I'd prefer a full disconnect between tiers. T1 = T2, but you can only gain XP at T2 by crafting T2 items.

I'd personally prefer to then make crafters forget older crafting (maybe 2 tiers down) and be unable to craft them anymore, but I doubt that majority of people would agree with that approach.