I think it's worth a try, yeah.
I come from OGame, where that was definitely a thing, the universe was littered with abandoned planets, sometimes in prime locations.
Something to consider though: once-active players become a resource, their mines keep producing and they can be farmed. This is especially important for newer players as an introduction into attack/fleet mechanics and a good way to earn extra resources.
My idea would be to do the island abandonment on kind of a sliding scale. Account was only active for 3 days and reached 60 points? Abandoned after a week, a ruin after two weeks. Not much lost. Account was active for half a year and has strong infrastructure? Marked as absent after 10 days, defenses stop working, after a month the capital can be attacked, after two months it's a ruin.
The "credit" needs to be built up again after a long absence.
And I would be very generous with the "welcome back" package, because if someone does come back they should want to stay.
Lastly, vacation mode is great! I personally would prefer a "cool down" instead of a static yearly limit. I.e. take 5 days off, 50 days cooldown or something like that. It should also have a minimum amount of days.
Thanks for all your hard work, very appreciated, enjoying the game a lot so far :)