I'm amazed that people think that TLC and maintenance will prevent failure. It's like saying "I don't smoke, so I won't get cancer".
No, it's like saying "I will reduce the chances of getting *LUNG* cancer by not smoking", perhaps.
Maintenance does not miraculously make you immune. Of course not, and it would be silly to state that. However, maintenance gives you this:
1) awareness of dodgy elements in your gear, and therefore you can do something about it before you have a failure
2) when you keep things in order, they are less likely to fail. Not immune to failure, but less likely. Just like if you maintain your car properly it will run for years, but if you neglect it it will become a pile of junk relatively soon.
You can't prevent every possible mishap. It just makes sense to avoid being in a bad situation *when you can* avoid it.
Like the example of batteries, something that a lot of people comment on.
You may have a duff battery. Yes, it can happen. It is possible. But it is a very rare event. Most people talk about battery failures which are simply due to not keeping an eye on it and not replacing it on time. This is very easily avoidable. Just on example.
I am not saying "don't use a back up, it's silly".
Ha! I have taken a spare bass and a spare amp, as well as extra DI box, cables etc... you don't need to convert me

But I have only done that at gigs that had to work well. In that case, the spare bass was tuned and onstage, and the spare amp head was also onstage, on top of the active one, ready to be plugged in and run should it be necessary. because that's another thing, I sometimes meet people who have a backup... back in their parked car on the other side of the building. Yup, it's going to be real useful there
What I'm saying is that a backup is a good plan in case something happens. But you can reduce the chances of anything happening A WHOLE LOT by maintaining your gear.
When to bother to carry backups etc and how you do it, that's up to you and the gigs you play. A bar gig, for instance, can easily tolerate your dropping out for a couple of minutes to replace a string if your band knows what they're doing. The music does not stop. The entertainment does not stop. THAT is unprofessional. But a good band does not do that, they find ways around the problems and still play music and get the people having a good time.