Network Security, overkill?

Maybe this a pretty dumb statement/question but I’m posting to get folk’s reactions...

If I have a firewall doing my NAT to the Internet with dual WAN internet uplinks and then multiple computers and servers sitting behind it on the LAN side of the firewall... are those computers really EVER at risk of random infections, viruses, malware?

Let’s assume no user ever checks email on these computers, never opens web browsers, etc. Is there honestly a security risk to just having a basic setup like this?

Are there really ways for intruders to reach my computers even though they are sitting behind my firewall with all ports closed inbound?

Thanks