Oh I agree completely - all kpop companies, even BigHit, treat their idols like a product to be marketed. The difference is that some companies are kinder to that product, have real-life attachments to that product, and some are completely impersonal towards them because they believe that they can be replaced.
Let's talk about TVXQ, for instance. one of SM's biggest victories and failures. in 2004, a year after their debut, there was a huge scare that SM were implementing a "rotation system" - essentially what they're doing with NCT now, and something they tried with Super Junior right after the scare died down. It started from a video of Yoochun crying during a performance, and it was spread that either Jaejoong or Yoochun would get replaced by a different trainee.
The scare escalated so fast that TVXQ needed to have an interview right on the streets, surrounded by hundreds of fans, to assure them that the rotation system was a myth. to this day it's unknown whether SM actually planned it, but 5-1=0 became kind of an anthem among fans. it was "all or nothing". That was the first instance of fan solidarity that could potentially make a company change their minds.
Of course, it didn't last, because SM weren't paying them anything, and JYJ broke apart. But with that came vicious fans on both sides, attacking every member, blaming each other instead of the capitalism that had led to them wanting to leave in the first place. I still firmly believe that this could've been easily resolved if they had been actually paid off, but they weren't. And people use this instance - their slave contracts - to claim that SM were mistreating them, and being in SM isn't a privilege.
But it is. Like you said - what is important is discerning between them as a product and as people. The fans, when they brought up that 5-1=0 banner, considered them as a group, as real people who had come together in a dynamic they liked, rather than random people who could sing and dance and harmonize well. The company wouldn’t have considered that - they didn’t have the same attachment to them. This is the same reason, I believe, that SM doesn’t do much about sasaengs. TVXQ’s and Super Junior’s sasaengs were a vicious lot, and the situation kept getting worse over the years. EXO’s sasaengs are almost as bad as they used to be, but there’s slightly more stringent measures in place. However, the company doesn’t sue them - because after all, they provide the fodder to SM’s cash cows. The whole boyfriend fantasy is marketed specifically to this kind of crowd, so in the end SM is the one providing the bait for these stalkers. It’s fucked up, but that’s how the world works.
At the same time, these particular groups are also catapulted towards the top. SM, JYP and YG groups had stellar promo going for them when they first debuted, even being able to perform before senior acts on music award shows, getting radio shows and variety show appearances. This is the kind of promo a smaller company would never be able to afford - or they wouldn’t have the connections or pull in the industry to get there. An example being - SM could’ve easily promised more appearances for EXO on a particular channel to get NCT a certain slot. Back in the day, they would have used the pull of Shinhwa and H.O.T. to promote TVXQ. GOT7 were touted as the “mini 2PM” when they first debuted. These acts all debuted with a strong fandom. As for YG, despite Blackpink being on semi-hiatus constantly, all their comebacks so far have been very strong digitally internationally, and the last one also made a pretty big impact on Korea. When they first debuted, there was already hype around them because of the mere fact that they were from YG! Heck, even the latest Billboard article on rookie Kpop groups to watch out for this year mentions the fact that YG is supposed to debut a new girl group this year, and we don’t even know for sure that it’s happening. That’s the kind of promo you don’t get unless you have connections in the industry, unless you’re rich and well known enough to make it
That’s where the difference comes in, and it’s really not that hard to remember. Any group that debuts in BigHit now will have that privilege, along with the burden of living up to a group the entire world recognizes. But BTS didn’t. Neither did H.O.T. or Big Bang. TVXQ didn’t have any precedence when they made a name for themselves in Japan either. In that way, that makes them exempt from the privilege of the Big 3, because they made it without any predecessors who would make the way smoother.