A cliche answer I know but the failure of the unite the right rally in Charlottesville was a major blow to them that they didn't recover from.
Once it became clear that the cost of openly supporting the alt right was that you'd be hunted down and physically attacked by police officers and antifascists, and were given a front row seat to what that would look like, a large chunk of the alt right thought it wasn't worth it to support them.
The alt right was allergic to action already, it was hard to organize anything other than memetic warfare, and fighting a politcal fight with memes only is like fighting a war with only your air force or more aptly with just your navy, you need boots on the ground to actually take things.
Additionally, the alt right community (this site being a prime example) was heavily influenced by the dumbest members of the group, and so were like puppets to the feds and Lea and most importantly to the alphabet agencies that glow in the dark (CIA niggers).
The history of the alt right was that there was old /b/ (03-13 RIP), and on there we had a theme of just debating and arguing over everything that was off limits for discussion before, from gaytheist vs christfag to cutfags vs jewdicks, to europoors vs americunts (murimutts), to propedo vs antipedo, MRA vs feminist, to all other sorts of subjects.
Eventually a meme emerged where people would troll by taking the most seemingly indefensible position they could come up with, and make the best case they could come up with to argue for it in all seriousness.
That obviously included defending the great Satan of the time which was of course adolf hitler and his Nazi sozis.
So the thing is, in arguing for a very taboo position that is not often given serious arguments for due to its unpopularity and controversial nature, you surprise yourself and go from thinking there's no possible way this would not be a stupid position to take, to realizing they not only have good points to make, but might actually be right about everything.
You see up to that point they were showing us things like american history x where the actors who hate Nazis were pretending to be strawmen of the Nazi position and setting it up just to knock it down in a film sponsored by the kikes in isreal to combat antisemitism, and the fake Nazis were the only exposure we got, and we assumed they were accurate depictions of them and their philosophy.
We were shocked to find that they weren't and became a bit obsessed with exploring their true ideas even further, to the point where M00T had to make /pol/ as a containment board, just to clear it off of /b/.
Its not a stretch at all to say that a bunch of suburban white kids with progressive left wing libtard views had radicalized themselves to the polar opposite of that position on an image board for anime discussion just by being allowed to talk freely at one another without their identities attached to their words, conversations constantly getting 404d and restarting, and without any federal agencies or paid shills interfering to derail the discussions.
As a result of this, /pol/ eventually became a political force, and started attracting all sorts of attention after /b/ was causing chaos IRL with things like chanology.
The modern alt right is boring, they became what was before /pol/ or /b/ existed, and that was a bunch of people who felt they knew what was right and opposed anyone who said the wrong things, you get kicked, banned, vanned, dismissed without an argument, they just call you a Jew or a fed and that would be it.
The kinds of no limit passionate debates that made the online movement great are gone, dogmatic thinking had seeped in and solidified their minds so that new ideas were verboten, as a result they lost the flexibility and thinking capabilities that made them strong.
So scared are they of subversion that they've done the subverting of themselves.
As for real life action, that was never their strong suit, they were terrified of organizing IRL, and as we covered this was the thing stopping them from moving forward.
I suggested a mass migration out of the western first world white countries to some shitskin hellhole where they could form an enclave outside of the jurisdiction of pax judeica and build up themselves in numbers and in financial resources, but that idea was shot down, they weren't able to do anything because they lived under the panopticon of the bankers but they would not want to take the hardship of leaving the range of surveillance.
So there was nothing to do, except become domestic terrorists, or off gridders and good luck with that, they would get infiltrated and shut down, or they would get Ruby ridged, early into their organizations.
This is just me dredging up memories of the good old days.
Now everything and everyone on image boards are newfag cancer, the chemo failed. Imageboards were a product of their time and are practically speaking dead.
By which I mean original content is gone, original thought is forbidden, social conservatism from the mainstream normie culture has infested us, and I don't think we will be going back to being a strong independent mover of the world again like we were before.