I mean, Stuxnet is an illustrative example that has been seen in the wild (and the rare exception of one that became public - in general, if you'd be the target of something like this and found out, the results of that analysis would be classified and unpublishable), and there have been almost 20 years to do improvements since Stuxnet was first developed, so there definitely are real attacks aiming to do stuff and/or exfiltrate data from air-gapped computers after "installing a virus on the computer" - and it's quite clear that Stuxnet did achieve a significant practical effect.