If you want to fund content updates for MP through MTs fine...
But don't do it through random chance.
Don't do it from making the game grindy.
Just offer cosmetics people want, and sell them to them straight up.
And yea, definitely keep that shit out of SP unless you plan on giving the ACO season pass content for free(which they definitely don't).
Problem is there are very few publishers who are genuine with this, they might say that's the case but the way to make the MTs is actually done with the intent to make more money then content expansions of old made them.