I've been at a mega-corp that let newly hired developers to build new stuff while senior devs are busy putting out customer fires that in many cases, were caused by those junior devs in the first place. "We want to make sure the junior devs have interesting things to work on so they stick around."

Not really. A new system means it is not in production yet, so if something goes wrong there is little impact. Breaking the working system doing maintenance can cause lots of problems, so it is seniors who are tasked with changing it.