This is a common sentiment that I really don't understand or agree with. If your goal is to make M, that can be accomplished with any platform. If you are already acknowledging that there are better guns (for you) than the Glock, why not upgrade , if finances allow, to make your progress easier. Practice is still paramount but C/D shooters will benefit from nice triggers, better balanced guns, etc.
I am not advocating every new competitive shooter buying $6000 Limited gun but the theory of "Buy once, cry once" makes sense to me.
If you love Glocks, no reason not to upgrade one--sights, trigger, stipple. However, the price point to enter the world of steel framed Limited guns is not that much more expensive. You can buy a CZ TS or Tanfo Elite Limited for ~1100 (less if used). There are some nice deals right here on the Forum