Okay, so it took the TF2 team well over a year between MyM and JI to create 5 weapons. (Dragon's Fury, Gas Passer, Thermal Thruster, Hot Hand and Second Banana.)

One of those weapons was horribly overpowered for 6 months before they made it more reasonable.

Now imagine if they released those 5 weapons one month at a time, with an additional 7 just for the first year after MyM, along with everything that came with JI and later on the Smissmass 2017/March 28th updates.

The TF2 team are horribly overworked as it is, we don't need monthly weapons that will probably be either complete garbage or absolutely busted until it gets nerfed.

Ideally Valve should be improving weapons that see no use, like the Volcano Fragment, the Sun-On-A-Stick and reverting the changed hitbox of the Bison. (Also maybe revert the Panic Attack too since it's now just stock but gimmicky rather than its own beast.)

But at this point, I think they're struggling to do anything but make cosmetics based on Steam Workshop and updating the localization files, so I doubt even a few good balance changes would be within their skillset. (Then again, JI did do a lot of good for weapon balance.)