I'm not sure what "them" exactly refers to, here, probably "the Paris Catacombs" in the title… which is not very precise. The misconception is that there is a plan to fill up a) the publicly accessible part of the network (the actual "catacombs"), or b) the non-publicly accessible parts of the network (the "carrières" and "abris" that both cataphiles and casuals incorrectly call "catacombs"), or c) both with concrete.
The publicly accessible part is basically the ossuaries, which is a very popular tourist spot. That part is profitable and very well maintained so there is no reason whatsoever to fill it with concrete.
The non-publicly accessible parts are the ones the cataphiles care about. _Many_ parts of it have been clogged with concrete over the past decades for the reasons already mentioned, but there is no plan to do that at the scale of the network which, given its size, would be insanely costly, challenging and pointless. No one is planning to do that.
The whole network? No. Just no.
What will continue to happen is concrete being poured here and there whenever the IGC identifies a security risk that can only be mitigated that way—entitled weed smokers be damned—and the cataphiles being pissed about something they can't do shit about.
