The Thick True Parabolic Mirror has a wavefront error of λ/24, which is 6 times better than Diffraction Limited Optics, and which is not diminished by the mirror's supporting cell, giving it the largest distortion and coma free area for its size, and making it the clear cut winner.
So with all these advantages, why isn't this the defacto standard? It is much more difficult to make in terms of time and effort, so the less difficult Paraboloid becomes more cost effective. This is especially true since the Paraboloid's failings are not readily apparent to the vast majority of the novices and amatures who use them. All you need to do is to teach everyone that coma error is an inherant property of all Newtonian type telescopes instead of admitting it is an inherant property of the more cost effective Paraboloid you put in those Newtonian telescopes.
This marketing strategy has been so successful that over the past few decades, many professional Astronomers have also come to believe that lie.  The 14" Thick, True Parabolic Mirror is unique in that it is the largest practical thick mirror possible, while being just large enough to be scientifically useful.  Used in a 12" Clear Aperture Newtonian telescope it will gather more light than any other Newtonian of its size. It also collects more light than any kind of Schmidt or Maksutov telescope of its own size.