It’s important to understand why mono-stacks can work so well. It goes into the math of the Damage Calculation.
In summary, each tile of damage has an expected damage amount equal to:
33 * (Attack / Defense)^1.35
Where attack and defense are fully modified by troops and specials. When you have stacked heroes, their cumulative power is added up inside this equation, e.g. with a three-stack:
33* ((Attack1 + Attack2 + Attack3)/Defense)^1.35
(It appears that there is some linearization of the damage as the cumulative attack gets much larger than the defense. More research needed).
A great deal of the benefit of stacking would be removed if the cumulation was outside of the exponent, e.g.:
33*{(Attack1/Defense)^1.35 + (Attack2/Defense)^1.35 + (Attack3/Defense)^1.35}
In my view, simply changing this math would greatly help the issue without imposing an artificial limit on using stacks.