An average L7 at Amazon will see that "limit disruption to Y and Z" is simply a constraint to the engineering problem and make it work. That's sort of what makes someone at that level - the ability to detect the cost for moving different constraints and being able to demonstrate the technorganizational advantage from doing it.
Combine that with the ability to get grassroots implementer support and you have a pretty able leader.
Everything is just constraint solving in the end, and the hard part is maintaining pace.
Of course. Implied in the senior developer's solution ("migration") is a plan to navigate those challenges - technically, politically and socially.
What I tend to see is ignorant management insisting that there must be an easier way, rejecting all empirical evidence to the contrary, even going so far as to reject the concept of engineering tradeoffs entirely. It's as if the experience and accumulated wisdom of the senior ICs is valuable only insofar as it makes them slightly more productive code monkeys, merely speeding up the existing plan put in place by management without any say in said plan. Why even hire senior employees at that point?
Combine that with the ability to get grassroots implementer support and you have a pretty able leader.
Everything is just constraint solving in the end, and the hard part is maintaining pace.