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Which $15 lenses are you referring? Are you talking about second hand lenses? Not sure any manufacturer selling new lenses is trying to compete against older lens that may still be available on the 2nd hand market.
In the current lens department, there aren't too many options available for Canon other than the Sigma 50mm, and it's own rather old (and rumoured to soon be replaced) 50/1,4. The rest seem to all be $800+. When/if the "midrange" 50mm replacement comes to market, it's likely to come with IS like it's most recent primes, and be up around the $800 mark.
Samyang sells manual-focus only lenses at extremely low prices. Cheap, used, manual-focus lenses are definitely competition for their market segment (unlike where current Canon/Nikon/etc. are aiming), along with new lenses from the major manufacturers. Samyang has done well so far by producing designs that fall in the rare intersection between low price and top performance.
Even discounting the used market, there are already reasonably-priced (not "$800+") contenders in the manual-focus 50mm lens space, e.g. the Zeiss Z* 50/1.4 ($725), Voigtlander 58/1.4 SL-II ($599) --- compare this to the situation with 85/1.4s, 35/1.4s, 28/1.4s, or 14/2.8s, and you'll see that the 50mm range is a lot more "crowded" than where Samyang has previously targeted lenses. It would take something awfully good to attract people away from the manufacturer's offerings, Zeiss/Voigtlander solid quality/reputation, and used offerings for bargain hunters.
Samyang could probably do something interesting here if they tried (extreme aperture and/or unusually well corrected), but my guess is they will pick more "low hanging fruit" in less crowded areas first, e.g tilt-shift lenses and other fast wide primes (how about an 18/2.0 for between the 14 and 24?), or maybe a slower supertelephoto.
P.S.: my Rokkor 58/1.2 works awesome on my Canon 5D (though mirror clearance is tight). My Oly, Nikon, Takumar, Zeiss/Contax, Leica, and Mamiya 50mm-ish lenses also reach infinity just fine.