I have a little over 500 accounts tied to my Gmail over the last two decades. Moving them is a pain but still possible.
The problem arises when it comes to existing accounts with byzantine financial institutions. Trying to change email addresses that are linked to you as the username is nigh impossible in many cases.
Quite a few institutions won't let you use an email that comes from protonmail or a custom domain. So, many times I'm forced back into using a Gmail address to prevent a whole host off other issues