I fully understand where you are coming from and I think that this is a good idea. I know that Skyblock isn't just about having island neighbours, but it does make the gameplay quite a bit more enjoyable.
Right now, on Skyblock, I am surrounded by inactive islands that have barely anything other than a tree cut down. This was also the case on /Classic.

I think that so long as the islands match certain criteria, they should be kept. Making this the block count (not overly high) for the island or the value of those blocks, like you have said, seems like a fair idea. As an add on to this, maybe even having a warning come up in their chat when they are creating an island so that the player is aware of this criteria.

However, there should also be a fair amount of time before the island would be recycled. In my opinion, 18 months seems reasonable, as this would allow the player time to come back and possibly start playing again on their island.

Just adding on to what you have said. C:

For now, I am going to go with neutral, because I think some more thought needs be put into this particular subject, but I am leaning towards support.

 

Maybe it should be considered based on the island's level?

 

Support. I think maybe a year or so after the person is inactive would be good enough.

 

Imperialism in skyblock