Dealing with existing code and moving it forward in significant ways without taking down production is always much more challenging than writing new code, whatever form those new ways take.

You can get by with one strong lead defining services and interfaces for a bunch of code monkeys that write what goes behind them.

Given an existing monolithic codebase, you can’t specify what high level services should exist and expect juniors to not only make the correct decisions on where the code should land up but also develop a migration plan such that you can move forward with existing functionality and customer data rather than starting from zero.