Achievements Connected?
I wouldn’t count on it. Flame & Frost did that, but they’ve significantly reworked how they do Living Story releases since then, and I think it’s far more likely that each one will have a unique meta-achievement.
The problem with connecting them is that if the number is too low, players who have completed the previous release will finish the meta almost immediately, and if it’s made higher to compensate, it becomes impractical for players who skipped the previous release. It’s much cleaner to keep them separate and give them separate rewards.
Everything Redenaz said.
Especially considering the fact that LS dailies now exist. If someone has done the daily every day for the entire Tower release, that’s an extra 14 “points” toward the meta achieve. They’d have to make the meta-achieve require over 26 (I think) achieves to keep anyone in that situation from just getting the achievement the second the patch goes live. And if it required that many, it would be a hugely daunting task for anyone who hasn’t done the Tower achieves.