I'd say there needs to be a change made to what constitutes an active account. An account should have x-amount of logged-in hours in the past year to be flagged active, not just "log into it once and you're good" like last time. No goddamn reason people should be sitting on 200+ character names, especially when you know they're probably squatting them to sell.
I'm relatively sure you'd get the boxes again on a new burn, but with nexon you can never be quite certain. I would suggest, yes, that you keep it around for legion, as hero is a pain in the absolute ass to level.
Plus side, if you've done the 5th job advancement and the proof of power quest(the one that has you go kill 10 happy erdas then gives you a starter arcane symbol), you should have been given a box that can be opened after July 22nd and gives you a mega burninator and a character slot expansion coupon.
The burninator lets you apply a mega-burn(two bonus levels up to 150) to any character, be it newly-made or pre-existing. The downside is you don't get anything else, no frozen set and no free snail pet. Do with this information what you will.