yes,you said so yourself,alternative describes a music style and
underground a culture,commercialism is not a music style,
you could say its a culture by itself;)

There is 2 things I learned about this Thread:

1. mainstream pretend to be underground or smh
2. There is some deep corners in underground that never gets mainstream, even if it gets released to the public and people love it, still it won't be felt to the fullest like its supposed to be 100%.

I can understand that reasoning, and agree with that.
That's why I posted about the similarity of the songs in this thread.
Since underground is not a genre of music, why are most of the tracks pretty similar. That was the thought process and still is.

Also the fact that under that definition many of the tracks posted here are deffo not UG.

lets take an example from the shahmen tracks i posted,
would you say they are not underground since they got a lot of
attention from vine videos or that they were not underground to begin with?

Dann, didn't think I would see Shahmen mentioned. He is definitely my favorite underground rapper, and one of my favorite rappers altogether.
My favorite songs by them are Mark, The Crow Pt. 2, and Get Mine.

I'm glad he's getting some exposure from Vine videos, but that definitely doesn't make him mainstream. Aside from the three songs you linked, nothing by Shahmen has received more than one million views om YouTube. There are a couple other underground rappers who received some decent exposure from people using snippets of their songs in short clips, such as Xavier Wulf's "Psycho Pass" and XXXTENTACION's "Look At Me!"

Mainstream isn't only the amount of views of a singular artist. It's the collective music tastes of the masses. A non-popular track can be a mainstream track because of the style it is.
People who make slightly sub par pop music are still making mainstream music, because they are making music that would appeal to the collective mass music crowd.

To call an artist UG, would mean to make the distinction between what the masses listen to, and the artist. And so far almost all of the tracks posted here (with exeptions) would fall somewhat under the mainstream tag.
f.e. twinsanity you linked is definitely UG, since the style of music he does is not something that is in the collective mainstream, while on the other hand suicideboys would be quite mainstream.
If you agree to that definition of UG and mainstream then you would also have to say that some of the music posted in this thread is quite mainstream at parts, and the fact that some of the artists aren't famous doesn't make the music automatically not mainstream. Would you be able to agree on that?

Could we come to a conclusion that generalizing a term like underground is even worse a definition than normal genres?

a good way to determine if a song is considered underground or not is if you would see that type of music played on the radio or have massive commercial appeal. Most if not all self proclaimed underground rappers have all their music out for free, and if they do make money off of it, it is from concert and merch sales mostly. i think you're just looking at the term from a scope that is too focused.

Nas? Never heard of him before.