Comment by chickenpotpie

Because it's significantly harder to isolate problems and you'll end up in this loop

* Deploy everything * It explodes * Rollback everything * Spend two weeks finding problem in one system and then fix it * Deploy everything * It explodes * Rollback everything * Spend two weeks finding a new problem that was created while you were fixing the last problem * Repeat ad nauseum

Migrating iteratively gives you a foundation to build upon with each component