Lol, ok you don't know, mainstream is what the majority of the mass market can afford, you are referring to the high performance segment, most people have a budget for parts especially for CPUs are around $100.
As much as someone like Linustechtips tries to classify products by its performance bracket as 'mainstream', its actually dictated by price, and price is king of anything, I will keep hammering this home but theres a reason why the best selling PC game in 2016 is farming simulator 2017 with under a million units sold and being only number 67 on the overall rankings.
Let that sink home to what mainstream is and why console game sales utterly demolish PC game sales, most people can't afford decent parts or I should rather say, people don't provide good and broad enough information about the components and try to think I7s as being the standard and considered 'mainstream' even though its out of reach for most buyers.
Also you realise the R5 1400 is around $150 right?, thats a 8 threaded CPU for almost half the price of the Intel I7, the price is lowering, and prices do need to come down, otherwise you will be stuck with a losing battle with PC game ports, mean just go back to the above to understand why, devs won't produce good ports most of the time and some games won't get released at all because they know damn well it won#t sell much, which is badddddddd.
By the way, in regards to your remark 'you dont have to buy it if you dont want to', I currently own a 5820K
.