I see your point, but while the path to the Void may possible by closed with magic or techmaturgy, the undead will always be a threat, regardless of what land they hail from.
If you wish to destroy the Void without becoming a perfect mechanical being, then our hope lies not in waiting for them to get tired out fighting an immortal enemy but in joining forces with the Shadow Isles ourselves.
May have trouble with this idea, but
and
have shown through their choice in abilities on the Fields of Justice that working together with the living is not only a possibility, but preferred among some of the undead.
requires the living to devour souls. If Runeterra were to fall, he would starve.
would have nothing left but what little sorrow there may be among the 50 or so survivors.
Perhaps, to test your statement of humanity being lost, we should mechanize
and
, who --unlike other undead-- are known to have been human and living and harbored such strong emotional connections that they still retained those feelings after undeath.