I think the idea will be that the confluence of events that led to Chris getting his abilities and meeting and marrying Tanya as well as them gathering unique individuals like Declan, Stacia, the Suttons, Omega, etc. around them will allow a singular opportunity to defeat the Vorsook possibly once and for all thereby eliminating the chance for them to wait out until Declan dies. I would also point out that it was mentioned that powerful witches age much more slowly than average human with a specific example of Declan’s Aunt Ash being much older than her appearance and if Declan is orders of magnitudes more powerful then he would age even more slowly.
I hope it is not just them fighting waves of faceless Vorsook who aren’t really important in whatever hierarchical structure the Vorsook use to organize themselves but you may be right and only time will tell.