To the original point, the Onslaught (which is still the Crusade for all intents and purposes) is fairly alive and well eeking out a living in Northrend. So the Crusade as a whole has never died or ended no matter how many times we sack Scarlet Monastery. To use a probably out of scale but appropriate example, the Roman Empire didn’t end when Rome fell in the 400s, Constantinople stood until the 1400s. Different name, different cultures, same entity.
The Crusade in the Eastern Kingdom though, as others have said, is a radical sect that had created a name for themselves for their uncompromising fight against all undead, primarily. What I think made them stand apart in players minds is the wild west of what the Plaguelands was, the actual danger that was felt as you stepped into it back in the day. It really highlighted how game play complimented, and created, world building. They weren’t pushovers in the slightest. Their fall from radical yet, relatively, good intentioned organization to human supremacists and extremists due to their circumstances (of course aided by Dreadlords 
) created compelling villains. At the same time, the Argents were seen as relative nobodies hardly clawing back against the Scourge whereas these red wearing fellas had entire bastions under their control. They were objectively successful to an outsider.
So creating them as a main villain isn’t as bad as others make it out to be. How they’ve been reintroduced, however, is their real weakness. We had the perfect opportunity to show how the strain of the world causes these folks to pop up, yet we’re never shown. They just appear.
What can be considered folk heroes in some way (how far does the whole true story of the Scarlets go?) could/should have had real successful recruitment drive in the wake of the Helm of Domination being broken and the remaining Scourge having this huge sudden resurgence all across the world. Plagued individuals in cities all over again, undead running rampant, a human kingdom who’s draft policies was about to call up farmers really about to scrape the bottom of the barrel, the Argent Crusade, who’s promised to take care of these undead things seemingly wholly ineffective of containing or stopping this from happening, of course people are going to start looking elsewhere. Who else is there as an alternative but this reactionary extremist group who knows how to lie and pitch their narrative? Especially one that can point a finger at the former Warchief of the Horde and leader of the Forsaken - of undead Lordearon as the reason for their plight. “The Argents counldn’t put down on of their own!” as they point to Lord Barthalomew and Faol. “The Horde killed your X, and yet they are allowed to remain within the Argent Crusade?” You have Death Knights attacking, and mostly getting through, Lights Hope Chapel, “No undead is to be trusted! Even our ‘allies’”. Any reclamationist knows Capital City is in ruins (more than normal) and the Forsaken are on shaky ground at best. Even if they’re not radical Lordearon reclaimers, what better time than now to kick them when they’re down, as we saw? They’ve also had five years to recruit until we saw them again, not to mention all the time prior to SL. They clearly had enough to reclaim the Scarlet Monastery by the time the Ebon Blade rolled through, so they’re not as insignificant as folks here are assuming. Now with demons, void elves, werewolves, undead, and robots walking through the Alliance, its not a stretch to condemn them even further and draw in those who are weary of these choices.
All that to say they are compelling villains, there is a narrative justification for them existing in larger numbers, they never truly went away, and a lack of world building is what makes them so jarring at this time. Having real consequences to the world Blizzard creates in the form of these radical sects, with reason to exists other than “I heard titans and order bad.” should be encouraged because this is how it actually happens.