Well, if you implement hardmodes or a 2-difficulty system, where most player does the normal version and then they are done, where veterans steamroll through normal and then jump to do the harder version of the content. Mythic is good thing, but it should be a thing reserved for the veterans and not a natural path of progression as many players see it as now.
Also, having the hardcore community on raid-farming is a pretty good thing, aslong as the raiding tier is done well. This gives time for hardcore fans of the game to build communities, do boosting pugs for lower skill-lvled players and raise alt-guilds for themselfs. The multiple raids might even make it so that the veterans would enjoy going back to older raids to enjoy some nostalgia feeling I would personally say that i loved playing through Karazhan on my alts in TBC, even after i had cleared BT. It gave a nice feeling of going back in time to my noob days.
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Yet it is often a top-3 raid for many veterans and hardcore raiders. The reason for Karazhan being popular, is in my opnion, because it had a good skill ladder. Many people got the taste of raid progression through Karazhan and because you had to improve at nearly every 2-3 bosses, people got to taste how good it feels to see your own skill increase by time.