Kinda does hold back mp though. People sometimes complain about elites not being a similar size or shape hit box to spartans and thats possibly the reason why elites became unplayable eventually, (and this could also explain why playable brutes that have been asked for since halo 2 were never implemented as well). This also explains why elites were only for one certain game mode in reach before eventually being completely removed for halo 4/5. Instead of being altered to match the hit boxes and play style of the spartans, they were removed. Why not shrink the elites or brutes down a bit, maybe adjust their stance to have the same hit boxes? Things could've been done to have a more diverse and creative mp experience but as far as most people are concerned, they didn't want to interfere with the Canon biology of the other species and left them out of the game entirely.
It also explains the removal of powerups and how they are now equipment pickups. It looks cool and ties into a more believeable thing for a spartan to have, but it alters the gameplay in a way that isn't necessary. Some instances, it can even slow down gameplay a bit.
Elites not being playable isn't because MP became canon, though. They could have been the same size like in Halo 3 or larger like in Reach. That's not really dependent on lore.
343i has also stated that Elites train with Spartans under the Anvil Initiative, so there's more reason for them to be included than not, if one takes the lore into consideration.
When we look at Mini Slayer from Halo 4; the wackiness of Fiesta Slayer; the pizza and fry weapon skins; the Timmy, Olive, and FOTUS armor from Halo 5, I don't think 343i are as much sticklers for canon as is thought.