The problem is insecure PCs and servers are a net negative for the Internet as a whole.
I'm now on a fast enough connection with good enough hardware that if you were able to get into one of my desktops or even just the router I'd be inadvertently sending out a lot of spam or "donating" a lot of bandwidth to a DDoS.
Also, scans of the entire IPv4 internet take less than an hour now, so there's absolutely no security-through-obscurity.
I'm now on a fast enough connection with good enough hardware that if you were able to get into one of my desktops or even just the router I'd be inadvertently sending out a lot of spam or "donating" a lot of bandwidth to a DDoS.
Also, scans of the entire IPv4 internet take less than an hour now, so there's absolutely no security-through-obscurity.