If not, then it's not an MBP competitor.
It may be a cheaper, have better specs, be better designed, be better built, have a longer warranty, or even all of the above, but if it doesn't run OSX, it's not a replacement for an MBP.
Of course, if OSX isn't important to you for whatever reason, why would you buy a Mac? That is their only real differentiator, and has been since high DPI ultrabooks began shipping from quality manufacturers. I do include 'need OSX-only applications', 'don't like Windows high DPI handling', and 'need proper color calibration across HW and SW' as 'OSX is important to you'.
Basically, a System76 running Linux is only competitor for an MBP for the very small segment of the market that want's a nice UNIX-like preinstalled on laptop hardware designed to support it and doesn't want a Dell developer edition.