Buildings don't decay because they are outside your territory (in 95%+ of cases, they sit on their own territory influence), they decay if not connected to a root. This would be unchanged. Territory ownership and connection to territory root are two different things. So nothing will change if you capture a CC and opponent have no other root attached to buildings => all buildings will decay. Your opponent will still own the "blinking" territory until the buildings are decayed to a new owner (unchanged and unaffected by proposed modification).

Your misconception here is to think I'm imagining something I'm not. I'm just trying to mentally emulate the consequences of the fix and point out anything unintended, given that you said "maybe I'm overlooking some potential drawbacks of doing so.. Anyone can think of one?". And I'm thinking of a couple of things, as I'll mention (I guess related to your "buildings are reasonably spaced" observation). Indeed "you cannot own a building but the land is to someone else" (except if they are decaying because of that). "There will be no more territory lines crossing buildings" is indeed the direct consequence of the fix, as is "buildings in the territory won't decay unless the CC is totally enclaved", that was your point all along.   I see now why there won't actually be "self-sustaining" territories: their footprints are not root, so they should decay if detached from root territory. But I was also thinking about this situation: I've had the issue that part of my walls would decay if the enemy territory expands. If I'm not wrong, this won't be the case anymore, since the fix wouldn't allow anymore for only a part of a long wall to decay. I don't have a problem with this, in fact I think the present behaviour is a problem (because my towers get distracted shooting the converted walls I want to gain back!), and your fix might have solved this. But considering all that, and, again, if I'm not wrong, it would seem that the fix would make buildings not "reasonably spaced" way harder to convert just by territory expansion since, given that "there will be no more territory lines crossing buildings", one would have to detach a whole not "reasonably spaced" group of buildings from the root territory. Maybe this is preferable for some, but seems a bit of a drawback for me.

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Why do the spammers keep doing it? Does it actually work?

There is no need for whatever complication you imagine. We just want buildings to always be in owner's territory. Which is how it should be, you cannot own a building but the land is to someone else (in this game at least). If this land is not connected to a root, then all buildings on it decays still, like what happens commonly in games when buildings are reasonably spaced. Basically in the picture with the CC in the comment above, the territory line would be between the CC and the barrack. There will be no more territory lines crossing buildings, which makes sense. None of the decay conditions are changed, beside that now the CC center point will actually be in the owner's territory (as it makes intuitive sense) and therefore buildings in the territory won't decay unless the CC is totally enclaved.