Mods would never go away because when you burn one bridge down another is rebuilt. Modders would find a new loophole to leak mods back into the game. It may not be as stable and secure, but I've never known a game to actually mod proof themselves forever unless that community simply gave up and did not think it was worth the effort. It's the same vein as when game devs patch out glitches and the speedrun community finds new ones. As long as there is community will, there is a way.
Now for fun theory crafting sakes: This game would experience probably one of the, if not the biggest earthquake it has ever experienced if Mods went away (including Discord though I highly doubt they'd erase third party sites, especially ones that they themselves use). If ACT went away, it'd probably go back to how ARR raids felt where people were judged on their mechanical ability and not their damage. People would stare at the aggro bar like they used to, to think they were doing good DPS. The PF would fill up with static recruitment ads as well as Reddit. I'd imagine Reddit would still be permitted even if Discord was removed. The SQEX team has been known to browse the unofficial Subreddit. The world races would take a serious blow and quite frankly they'd still find ways to "cheat" for lack of a better term. The amount of people that are already fed up with the direction this game is going would quit the game citing that it was the final straw. It'd be very catastrophic. Every social media influencer would be covering this subject and it'd make global news. Linkshells would make a comeback, Free Companies might make a comeback. Fellowships would probably remain dead because without external cites like Discord they still serve no purpose. Venues would come crashing down and struggle to stay afloat. For most purposes, to put it simply, just remember what 1.0 and 2.0 were likely and the community would most likely become a similar environment to that except a bit more ghost townish since we're super spread out. The game would quite literally have its own exodus. Some people might gradually return a few years along the way overtime. But the population would probably significantly drop down to its old ARR/HW population. (Which is more than a 50% decrease). The reason it won't completely plummet is because of loyalists, purists who never modded in the first place. And console players who were unlikely tainted in the first place. The other thing is there is a severe lack of other games to play, so even though the population would drop, there would be some that stick around because there still isn't anything else to play, and removing mods isn't going to magically make another game sound worth picking up.
Depends on your definition of hardcore. Would you consider speedrunners in other video games as being hardcore? If so, it might surprise you that the speedrun community is plagued with add-ons. Not for their live runs, but to measure game mechanics and discover new ways to break the game. A speedrunner using a modified practice rom isn't much different imo to using ACT. (cooperate legality aside). I'd even argue that it's the hardcore players that make the add-ons in the first place because they know what kind of data they are trying to draw. A true casual would probably never know the difference between Direct Hit and Crit's potencies if a hardcore player didn't research the truth out. Often times, it's the hardcore players that figure out why some additions to the game are actually bad. If we were unable to graph and process data, we'd just live in a world where the devs make changes, tell us it's good for us, and we'd accept it without ever proving them wrong.