Quick observation on Lurrus, nothing new, just another perspective: I've been on Grixis Control for years, it's the only deck I've ever seriously played, apart from UR Thing in the Ice. And when Lurrus arrived, I stopped playing Grixis Control because I thought Lurrus doesn't slot into that well enough, and switched to Grixis Thing in the Ice with Lurrus. Had some ups and downs with it.
But recently I've incorporated Lurrus into an Into the Story
Grixis Control list running Young Pyromancer instead of Tasigur, and just ditching Gearhulk altogether. SB had to be adjusted, but there's even the option of not doing that and not revealing Lurrus as your companion.
Quick sidebar: I think thats one of the dumber things with companions, that you can choose not to reveal them and sideboard whatever you want. It should be mandatory.
Anyway, Grixis Control with Lurrus is insane. The restriction from my original list was literally 1 card (Gearhulk, and I was already on YP) and several sideboard slots (2 Kalitas, 2 Fulminator, 2 Ashiok), but Lurrus is just so much better than all that, and Nihil Spellbomb/Tormod's Crypt plus Lurrus gives so much consistent play against GY strategies, its just incredible.
And the best part is that Lurrus is a genuinely good card vs Burn/Prowess. Having a 3/2 lifelinker isnt that impressive, but having it every game is a huge deal and in the 30-something matches I've had against RDW, it steals about every 5th or 6th game vs Burn, no kidding.
So for my fringe deck specifically, Lurrus is just a great way to make the strategy 1) generally viable, 2) keep up with all the other Lurrus or non-Lurrus Companion decks and 3) shore up the single matchup that has historically made Grixis control struggle in any metagame with RDW.
I'm not necessarily defending Lurrus from being a ubiquitous card in our format, but there are some huge positives that come from having it in the format.
1) I really like the fact that Grixis Delver is a thing.
2) that Lurrus gives the meta a leg up vs Big Mana because they can't run it themselves.
3) that while Lurrus is good in Burn, in my experience it's even better against Burn.
4) that Lurrus + Crypt/Spellbomb is incredibly strong against Graveyard decks (Crabvine, Dredge, Living End, Rakdos Pyromancer lists etc).
If one has no problem accepting "reveal companion" as part of the game like "draw starting 7" or "roll for play", maybe it's ok to have these cards.
Lets look at 3 alternatives to leaving all Companions legal:
1) Ban just Lurrus: as others have said, the metagame would just tilt towards the other companions, which would make for another 2-3 months of bad modern an end with another ban, then another ban, and another until all of em are gone.
2) Ban all companions: modern had some massive problems before Companions, and we would just revert to those. I won't go into them again because there are a good 50-60 forum pages on the topic just a few pages back.
3) Adjust the rules: would be a very good option too imo, probably the best for everybody overall because people get to play their cards without having an unfair advantage.
I've been for banning Lurrus since it came out, but I admit I didn't try running it. I have now, and at least for my personal fringe deck, Grixis Control, having Lurrus stick around would probably be a net positive as opposed to pre-Lurrus.
EDIT: It's also not so bad for accessability to the format. Yes, Lurrus is a 15$ card, but you'll only ever need 1 copy if you're running it as companion and just having Lurrus (or other companions) will make a lot of budget-builds much more playable against all the non-companion tier decks.