Hey @aedroberts! Thanks for having a read!
Yeah, the witnesses are what keep Steem up and running for us. If they all decided to quit at the same time, we would be in trouble.
For many of them it is a full time job, and their only job. For a lot, they have a full time day job, and then put another 6-8 hours of their "free time" into Steem, plus all of their days off. For one such example I would encourage you to visit the following post from @sircork and listen to his interview with @timcliff who is currently witness #1 and has an additional full time job.