Wizard telekinetically shoves weak rogue ally grappled by kraken (str30)

"The rules aren't an attempt to model every possible contingency."

Forced movement doesn model ANY contingiency. You push, the target either purposefly fails thr save and is pushed or makes a str save against the pushers ability.

Its not like there are a dozen special cases to forced movement rules, and they just didnt want to add one more for pushing a grappled target.

"In that scenario, it's not unreasonable to assume that a sudden external push can break the grapple. It also gives players more tactical options if one of them gets grappled,"

Being grappled has been slowly getting nerfed over the years. This is just part of the nerfing. Its like "Surprise" doesnt do much at all now. Back in the day, Grapple and Surpise could quickly swing the fight against the players and even quickly lead to a tpk. But in the latest rules getting grappled has almost no direct effect on you. It only matters if there is a secondary thing that happens after the grapple. 

Like being swallowed. Or a mind flayer sucks out your brain. Both start with a grapple. The grapple itself doesnt affect you in any way that keeps you from attacking the monster. Same attack roll, same damage.

And even though grappling is almost completely nerfed into nonexistence, this rule means any ally can push or pull you out of a grapple, you voluntarily fail the save, and you are out of the grapple, regardless of how strong the monster holding you is.

"and is mechanically simpler, requiring fewer die rolls."

I dont know where it requires extra dice.

"Anyway, there's no clean alternative"

Sure there is. Monster grapples the fighter. Rogue tries to pull fighter out of grapple. The creature who makes the save against the Rogues Pull is the MONSTER doing the grappling, not the fighter.

Still only one die roll, just different ability score.