Also when you could just plug something into a switch and a) the port is active and b) it goes unnoticed.

Probably still the case in a lot of startups.

Most startups I deal with these days don't even have offices anymore, let alone network ports. =)

But yea.. same deal with wifi. Its amazing how often the wifi password is posted on something visible. In fact I have found the password in so many public images which means somebody on the street could just connect to the wifi network fomr the street.

That's only a denial of service issue, not a security issue. Wifi is too cheap to meter

Unknown persons getting onto the company wifi is not a security concern?

It shouldn't be. Treat the network as public, require credentials for any access whether it comes from inside or outside.

With work from home the security perimeter is the machine. All networks are untrusted.

Correct. Company wifi is just like any other network. Why would there be any services on it that don't use encryption and authentication?

In the context of this thread - how many startups do you think strictly adhere to best practices?

In this context, almost all of them? The whole concept of a "secure network" is very enterprisey