I forget that they are calling the Wolfspear a successor, which I don't agree with since they are basically a whole new "breed" then an actual successor in my opinion. But I digress, considering their role and title as a "successor" it does make me believe that they might have some additional options in terms of allying since the guard of the rift thing would be a multiple force endeavor.
As for our currently lost great companies, I just had another thought that maybe they would leave them unknown or unreplaced to allow players to create their own great character companies to their own means. Want to recreate one of the old ones with a new Jarl? Go ahead. Want to create a new Primaris great company? Go ahead. A hybrid SW/Primaris? Go ahead. It opens up the option of no longer having to relegate your great company to being one of the 13th who left the Fang or renounced their oaths or whatever and allows them a great company on the grand annulus.
Agree that replacing the depleted great companies does provide opportunity for adding primaris marines to the chapter. You can base tactics around the new units or a combination in the same way that existing great companies reflect their wolf lord's preference.