Fury's claim to the lineage in case AJ/Wilder fought in March 2018

I guess if a boxer has a "lineal" claim, he has it for as long as he doesn't lose, even if he retires. He can come back and claim the championship.
But that's not to say another boxer can't be regarded as having starting another 'new' line. And that part is more subjective.

It's worth noting the "lineage" Fury claimed goes back precisely as far as Wladimir Klitschko.
When and how Wladimir acquire it is subjective.

 

My opinion on this issue if champ A retires, champ B is named and champ A returns you have two lineages. And champ A will either fight champ B or they will either retire again or lose to someone who will fight champ B. Either way the situation usually resolves itself shortly and you've got one lineage again. Though this generation of ducks would have challenged that logic. Fury being only 27 when his absence started had the potential to be a pain for a much longer time. A potential he realized despite his return.

The Ring were in "not this **** again" mode after the whole Vitali Klitschko thing and were going to wait as long possible before replacing Fury. Who had yet to turn 30 and had been gone less then 3 years. With Tunney, Louis, Marciano and Alis exile whether or not the old champ was coming back the Ring waited for all other claimants to be out of the picture. In 1930 while the IBU still had a champion they had named Schmeling champ and had stripped both him and Uzcudcin without losing and Schmeling had beaten Uzcudcin. Holmes eventually beat Ali but before Alis return they gave him the belt once Weaver became the WBAs champ as Holmes had beaten Weaver.

In March 2018 Parker was WBO champ and Luis Ortiz was an undefeated former interim champion then AJ and Wilder beat them. March 2018 was really where the conversation to replace Fury would have started not when it would have been done. And of course Fury returned shortly after. If Wilder and AJ fought in March 2018 then Parker would be still be WBO champ and Ortiz would still be undefeated. While AJ v Wilder was strong enough to be for the lineal title the Ring was mainly waiting for all other alternatives to be off the table. So even if Fury were to return then its just one matchup.

 

Wilder was the guy with something to defend when he fought Fury anyway.