Bear with me here for just a moment, but what if companion was meant to be a new game space that WOTC is exploring. They have been making efforts in the past year or so to cushion the mulligan phase and this does go a long way to make mulligans even less painful. This could just be the first wave of companions mind you, what if there are companions, or something competing directly with the game space they occupy in the next few sets? Spells with companions, Mono colored companions, Mox companion, etc.
Purely theoretical mind you, but my question becomes what if Lurrus is knocked out of being dominant by virtue of there being enough other cards in that space, or likewise something gets printed that would preclude you from using Lurrus but is powerful enough to want to play instead. What if Lurrus is dominant by sheer virtue of him being the best cards in that space that is only occupied by 10 cards right now?
WOTC certainly would not ban the mechanic if it is something they have been planning and building around already in future sets, and banning him would seem stupid if suddenly there are 2 or 3 other companions of comparable playability and power level.
To my point. What if companion mechanics were just a given from here on out. Basically regular tourney magic became a format where everyone had basically a vanguard card of sorts. Sideboard was not always a factor in Magic, neither were pregame effects, but now we just take them all as part of the game. If we knew that was the future of the game, would we still be having this conversation?