That’s what I assumed originally, too, but that’s not what Alex said about it in a dev stream from 2019:
If you click through to the source of this quote, you’ll land in Multigun’s recap post where you can get the link to the video and listen to the question and the answer yourself.
I like to assume that they’re not lying, so that means that the purge is not a server cleanup mechanism. It might have originally had that in mind, but that’s not its purpose anymore and hasn’t been for a long time now.
This is why the suggestion to add a building upkeep system is a recurring one on these forums. Even the devs recognize that the decay system is not quite enough. Fortunately, the devs haven’t caved (yet) to the demands to put a hard cap on the number of building pieces. If they have any plans to address the “overbuilding” in the future, I really hope they design a solution more elaborate than “lol just slap a hard cap on it”.