I think that almost irrespective of how beacon persistence is handled (and more specifically, removal of abandoned beacons), there's something that only seems to have been mentioned in passing and not really considered that weighs on my mind based on how I know I'll end up playing.
I can imagine that I'll play a lot for when the game is launched, and really enjoy it. At some point however, it will start to feel like a chore, or some other exciting new game will come out that I want to try, or [insert other reason here] will make me put Boundless down for an unspecified and not entirely foreseeable amount of time. Given that Boundless is going check most of my boxes for fun gameplay, I don't think I'll ever just stop playing with no intention of coming back. However, if I knew for certain that without constant* upkeep I will lose everything I've collected, it will be a real turn off and disincentive to come back to the game in the future.
I'm very much in agreement that unattended beacons deteriorate over time (or without some from of upkeep) as a method to keep the land clear of abandoned beacons. However, without some functionality to save your belongings over a longer (potentially indefinite) period of time, I'd suggest that it will be VERY difficult to bring past players who take an extended break back into the game, which would be a great loss.
It's not a fully formed idea, but I feel we could really do with some method to essentially 'pack up' a beacon (collecting all the blocks that you've added or modified) and putting it into an escrow somewhere that won't disappear over time. Then, when you know you're finished playing for the foreseeable future, you can pack up your beacon and release the land back into the game for someone else to claim. The important point, is that you'll know that when you do come back, you won't be starting from scratch (Yes, except coins and experience).
This should be of benefit both because it will provide an improved avenue for taking longer breaks (and people WILL want these at some point), but also that in doing so, you can apply stricter/harsher time limits on how quickly it takes for unattended beacons to degrade, for the very reason that there IS a way to circumvent losing all your belongings if you're willing to look for a new home later. And lets face it, if I've been off for 6 months, I'll barely remember or care about my previous beacons, and will want to build something new anyway.
*Constant upkeep, in this case, could actually mean anything. I feel that when I play a game, it's because it's my free time and I want it to be fun. If it starts to become a chore just to maintain my property, then the game has lost it's ability to simply be fun. If it isn't any real chore to maintain property, then whatever IS set up as the requirements (be it log in recency, or inexpensive fuel) will be something that people can do without much thought, and it fails to be a deterrent or preventative for grabbing land and not using it for all but the completely inactive.