Naturally, it can take time to get parts, so if something is actually broken, then the bike is out of commission for a while. But a minor benefit of poor urban planning is that I've got space for a spare bike or two, or three, plus a bucket of spares that I've collected over the years from trashed bikes. ;-)

Also, doing the work at home means that there's less of an impediment to keeping the bike in good adjustment more or less continually, so parts don't come loose and start to deteriorate. Major repairs become less frequent. More or less weekly, I go down into the garage and check all of the bikes in the family fleet. The effort is practically trivial. I also involve the kids in this work, whenever possible.

With an eye towards doing my own maintenance, I choose my battles when I get a new (or more likely, used) bike. I prefer parts that are likely to be reliable, and that are straightforward to adjust without a proliferation of special tools.