[2019] Whats the problem with new decay system ?

It's a client-side issue we'll need to resolve. Single/isolated foundations with structures on top of them are being triggered as 'fast decay' by the client, however, the server recognizes that they're legitimate and marks them as normal decay. Due to the mismatch between the client and the server, your game tells you that you can demolish it, however attempting to do so results in a failure because the server protects against it. The server always knows the true state.

It's a confusing issue and hopefully one we can patch soon, as it doesn't allow folks to know when they can actually demolish it.