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In the games I play that do it, the story Arc equivalent spawns 2-4 'tiers' of content, sometimes in different locations, and different players gravitate to the one that lets them be most effective/efficient.
One of the other methods used is to just count on player interest dropoff. Content is introduced at low level and if left alone will get stronger. For a certain period, the low level characters push back, when they lose interest or outlevel it, less people do that, and the content levels up to the next tier, and people of the next strength level become interested.
Eventually even when the whole server is strong, the situation is solved still, it just takes longer for the content to become challenging or available, but the ability for low level players to still engage with it and make some impact, remains.
This is for games with more 'seasonal' content strength/'Story arcs' though. The equivalent in the game that doesn't have this is a matter of hierarchy, so it would be the difference between 'saving your Node's building' (for low level players in lower level nodes) and 'saving your City's Reliquary from a beastman attack' (for higher level players in higher level nodes).
The High level players could easily move to just overwhelm the low level content, but time spent vs effectiveness is a factor.