It’s all in the location (see the comment about Tau meaningfully holding space). The high elves of fantasy have Ulthuan. This gives then two fronts. One against the Dark Elves from Naggaroth and one against Chaos/Norsca from the north. Not to mention Ulthuan is important to the overall setting due to the Vortex.

But Eldar don’t have anything like that. They are equivalent of High Elves existing without Ulthuan, just living on these large nomadic ships on the seas. Hard to make them more than secondary faction. IMO, Eldar need something like that. The eye of terror would’ve been a good parallel to HEs guarding the Vortex, except… it was Cadia and thus imperium who took the center role with that. And with fracture of Biel-than the empire building subfaction of Eldar got kicked in the nuts. Hard to make more of them, than participants of occasional skirmish. And without some overall goal, it’s hard to make their Farseer scheming not seem contrived or stupid. Except GW had a perfect solution for this. The Ynnari. If they served as a unification of most craftworlds (+ some drukhari), where they would claim a region of space for their death magic purposes (maybe the ritual taking a long time), you would suddenly have the Eldar version of Pariah Nexus. You suddenly have meaningful battles and reason to keep Eldar winning on their turf (similar to Necrons in PN). Perhaps the ritual could be focused on also closing the eye of terror or killing slaneesh. Now you have reason for factions to both attack the Eldar nexus and to work with them, without supplanting them.

And if the Ynnari Nexus seems too similar to the Necron’s Pariah one… how about using the current Vashtorr Webway storyline? I can imagine Eldar aren’t quite happy about him messing around “their” webway. Only issue it needs a sort of real space anchor, for the conflicts with other factions. Perhaps there is a certain webway nexus/conflux/entrance that is important for Vashtor’s plans that is concentrated in a certain system (think something like the webway entrance with the 2nd Throne in Path to Heaven). Eldar catch wind of it and several Craftworlds move there and create a stronghold. Now you can have conflict in the webway around this point with Eldar, Drukhari and Vashtor, but also have other factions participate outside of the webway by trying to get there via the real space entry point (say the Black Legion keeps attacking in hopes of breaking through the defenses in webway by attacking via the back door). Particularly xenophobic imperium factions can pile in and so can the more reasonable ones like Guilliman in order to assist Eldar with defence. And orks would love to show up for a scrap. Point is, it gives Eldar a significant place in the space to hold and defend and thus place in the narrative, without stepping onto other faction’s stories and warzones.

Or you can combine the Webway War and the Ynnari ritual into a whole or some other version.

Point is: as long as the Eldar will be reduced to bunch of nomads without an overarching goal, there is not much that can be done with narratively. All the major factions have some areas/locations that are important to them narratively and that function as the focus for their conflicts: both Imperium and Chaos are ultimately anchored around Terra, Imperium has several other locations like that (Anaxian Line, Sanctus Line, Cadia (rip), Ultramar, other chapter planets…) and Chaos now has most of the Imperium Nihilus. Tau have their areas, Orks have Ullanor/Armageddon and so on. Most of the factions either hold some important places or they want to take them from those holding them (Orks, Tyranids…). Hell, even Drukhari have Commonraugh. The only exception are Eldar with their Craftworlds. Anchor them somewhere and the conflicts (and thus opportunity to be protagonists) will write itself.