"Mainstream Media" especially in contexts like these is meaningless. You seem to mean many PEOPLE don't like them. There's an argument to be made that for the average person, being attacked by a number of Substack's writers would make that average person less likely to explore their other writers offerings. Fewer readers means less revenue.
"Mainstream media" is certainly not meaningless. A good proxy would be any organ considered an "authoritative source" by Facebook: NYT, WaPo, CNN, Politico, AP, etc.
As for "fewer readers" I don't know what you mean. The "readers" are paying their money, aren't they?
Meaningless in that the phrase is used almost exclusively in the way you use it, and yet you (and others) aren't able to construct a coherent definition. Regarding your examples, Facebook was a weird choice of authority when viewership/readership are easily researched numbers. I bring this up partially because the only thing that you believe is "media" is news sources but the most popular news sources aren't "mainstream media"? There is clearly something specific you're filtering for but are unwilling to say. Until you do the term means nothing.
Wow, you're really going to elaborate lengths of pedantry here, aren't you?
"Please define 'mainstream media' and no, I'm not satisfied with your definition."
Why is this important? You could make a list of 100 news sources and ask a random sample of 1,000 people to check Yes/No on "is this mainstream media?" You would get a very high level of agreement. It's not my responsibility to give you a definition you're happy with. This isn't a scientific debate.
Your definition differs from the results offered as evidenced by your examples differing from other examples: i.e. of The Big Five[1] you picked on only the smallest, not even focusing on journalisitic enterprises because one of the larger organizations literally called "News Corp" didn't have a single of its publications make your list of news outlets. Curious.
Exhibit A: Alex Jones. Whacky conspiracy theory and snake oil peddler. It's literally in nobody's interest he be published anywhere, apart his own personal financial one.
In most cases there's probably a good reason for someone to be shunned by everyone "mainstream".
No one is worth reading solely because the “mainstream media” doesn’t like them. People who jabber complete fiction are disliked in that way. Doesn’t mean they’re worth the time.
Glenn Greenwald in particular depresses me. The days of his blockbuster stories feel like distant memories, now he just posts poorly edited (or more likely not edited at all) opinion pieces powered by nothing but rage. I genuinely don’t get why people would pay for it.
You say that like it's a bad thing.
Being a contrarian means that the mainstream media doesn't like them, but apparently enough readers do that they can make bank.