Someone being eager to be a mod does not mean they'd make for a good mod. Imagine if HN was modded by a new guy every two weeks (eagerness doesn't tend to last and is not a precursor to perseverance in my opinion at least).
I haven't seen any statistics, but it looks like only a few well known mods are leaving. So, almost all mods are remaining.
There are a little over 600 total moods on StackExchange. Over 20 mods on the StackOverflow part of StackExchange. The announced departures from StackExchange are in the tens.
For better or for worse, SO will be able to scrape by with the remaining mods.
The only real problem for SO is the lack of trust. I've lost trust in the employees of SO. With every post from a SO employee, it gets worse and worse.
Who are the direct competitors of SO, if any? I would like to use a different site, if possible.
---
On this page which is updated daily, https://stackexchange.com/about/moderators I count 629 moderators across 173 sites.
---
For example, the Wayback machine shows that on December 9, 2018, the moderators on the StackOverflow part of StackExchange were these 26:
Michael Myers, Robert Harvey, BoltClock, George Stocker, Brad Larson, ThiefMaster, ChrisF, Flexo, Ry-, Bohemian, meagar, Martijn Pieters, Jon Clements, josliber, Madara Uchiha, TheLostMind, Undo, deceze, Aaron Hall, Bhargav Rao, Andy, Cody Gray, Yvette Colomb, Samuel Liew, Rob, Ed Cottrell
Today, I see these 23. If you remove George Stocker, 22:
Michael Myers, BoltClock, George Stocker, Brad Larson, ChrisF, Flexo, Ry-, Bohemian, meagar, Martijn Pieters, Jon Clements, josliber, Madara Uchiha, Undo, deceze, Aaron Hall, Bhargav Rao, Andy, Cody Gray, Rob, Baum mit Augen, Jean-François Fabre, Samuel Liew
Yes. That's a significant drop, but not enough for the users of StackOverflow to notice a difference.