Unified and lineal are two different things. Nobody considered Bradley to be lineal when he unified with Holt or Alexander. But everyone considered Vazquez-Marquez II to be to create a new lineage even though only 1 belt was at stake.
the true way to re-establish linearity is a complete unification like Hopkins did. But of course these days that is almost impossible to you have to be a bit more flexible.
Back when it was just the WBA and WBC, sure. But in reality, it's always been by popular thought. Lineal is a media creation. The sanctioning bodies didn't exist forever, but before they did people still knew who the real champs were.
To say that it's necessary to "unify all the belts" to be champ is like saying Lewis wasn't lineal champ in 2000 because he didn't have Vitali's WBO belt. The only sanctioning body that's relevant for determining the lineal title is the Ring rankings imo. The WBC itself doesn't have any authority. After Lewis retired the WBC title was fought for amongst Rahman, Barrett, Toney, and finally handed to Maskaev. But the best heavys at that time were not these but rather Wlad, Peter, Byrd, maybe Brewster. Granted in March 2006 Rahman was #2 and Toney was #3, but that WBC fight couldn't be for the lineal title if Byrd was #1, which he was until Wlad knocked him out in April. By beating the most feared guy in the division (Peter), followed by the #1 in the division (Byrd), following by beating the most dangerous other titlist (Ibragimov), WLad carved a new lineage. Beating Ring #3 Chagaev should have clinched it. Apart from his brother, Wlad has more or less cleaned out the division.
I think that purists are too rigid. The heavyweight title could be vacant for all the rest of the century. I think the Klitschko reign has to be taken into consideration. One guy dominated the division in 2004-2005 and then was the #1 contender between 2008-2011. His brother has dominated the division between 2006 and 2011. That dominance has to be considered someway.