It wasn't an upgrade to a new major version, I only do security updates until the software I use (in this case Xenforo) gets a new major version as well, as in this case I need to retest everything anyways.
No idea why, but for some reason the security update disabled the cache which was the reason the site was overloaded and lead to some issues. I don't really see a reason it did that... maybe a mistake someone made in the package, but it happened.
Almost all open ports you listed there (except for 2 or 3) are being used and therefore need to be open. This is a dedicated server that's used for web, email, git, ftp, etc.
Every single service is protected by fail2ban and similar and doesn't spit out detailed information that can could be used to figure out what security holes the server should have.
I regularly follow newly found exploits and check whether they apply to the server and software and close them.
I have regular backups (daily), so I'm not concerned of someone breaking in and deleting data. But I have tons of customer details in the shop, so I don't want anyone to break in and steal the data.
That's the reason I install all needed security fixes.
I doubt it's a good idea to not install security updates simply because the software is running good as is and ignore the fact that a lot of newly known exploits will find their ways into the next generation of bots trying to steal as much data as possible.