Mainstream to me is like what your regular person without sophisticated taste is able to swallow and enjoy, it's flexible (playable on any occasion), simple, it has to sell well just like any other product for masses, and somehow trendy so the majority can talk about it. Theres hardcore selection for overall flow, sounds used, words spoken, so artists freedom is very rarely in it, it's more like label is forcing what you will get to hear. It's been like that for many years, it's just we now live in a far different world than we had in 90's and before, we have access to information and internet, and music style thats pushed mainstream happens to be the worst since years, but that also goes for many different areas. At least we have choice in music's area and its all affordable, often cheaper to pay for original and non-mainstream content, unlike other things like electronics or food quality for example, which used to be good back in the day and now its worse and worse, even if more advanced giving a lot flexibility.