Interestingly enough, I've actually just started running Lurrus in any maindeck that can support him mana-wise, but not as a companion, and has a fair amount of < 3cmc drops already. That's in Modern and Vintage. And he's just great whenever he hits. He's just insane valuetown. I actually find his lifelink to be the most annoying trait as you can't come out ahead in any way - even surprise chumping with snapcaster as a 1-for-1 gives them a free healing salve. There's just no stopping this card from being awesome, whether he's companion or a main deck value-engine.
This is actually undermining to the argument in the first place. If Lurrus is better maindeck as a 4 of then it would go some length to show that companion is not the issue, it's the power level of the card. If the card is too powerful as a 4 of the first step would be to restrict it. What if the ideal build for Lurrus is actually some copies main deck with a different companion in the board?
I think you're misunderstanding me entirely. He is INFINITELY better as a companion. What I'm saying is that he is also a very strong 3-drop maindeck for decks that can't conform to his companion-mandated deck-building requirements. Like in a modern deck I run that is an elementals deck; it hinges on Lightning Skelemental and Spark Trooper, so I can't make it's restraints work for Lurrus as companion. However, Lurrus buys back Thunderkin Awakener and Flamekin Harbinger all day, as well as being a lifelinker, so he's just a solid include. That deck runs the Orphanguard as its companion - and Lurrus meets that requirement just fine. Lurrus is a very solid card maindeck in any quantity. He's extremely strong (far more than main deck) as a companion.
A couple other notes:
When you say "Most veteran players," I think you are speaking for yourself and a handful of people you know. Myself and many others I know are just fine with changes and realize they are inevitable in a continually-growing format. If bazaar or workshop got restricted, we'd embrace the shift, not bemoan it.
I also think you miss the point @chubbyrain1 was making. It's not that Vintage is defined as Lurrus combo or Lurrus value to compare variations of a build as you would say pitch dredge or sun titan dredge. Sun Titan Dredge and Pitch Dredge are two variants of a similar build in a meta where Jeskai xerox, PO, humans, shops, hollowvine, etc. are all viable options. What chubby's post suggest is that Vintage is now Lurrus combo/lurrus value as tier 1 and all else are piles of non-viable 60 cards in comparison (as far as the numbers and results thus far have shown).
I agree the cat is just the new flavor everyone wants to try and it's overplayed...but how strong it is over time we'll find out soon. What if the trend continues and Lurrus builds still have a 60% winrate vs ANYTHING non-mirror in perpetuity? What if it is refined over time and gets to 70%+? Are you still okay with WotC doing nothing to the mechanic so that Vintage stays "pure" even though there would only be one deck type vs chaff left as a format?