Building houses is a "more-or-less one-time event" too by your logic. I don't see the construction industry evaporating any time soon.

In addition, as someone else said, there's a LOT of roofs left to install solar on.

On top of those factors, these systems will surely need some amount of maintenance and repair; the installation companies can do that too, since they're already experts on that particular hardware.

On top of that, solar panels degrade with age, and newer panels are technically superior, so just like people replace cars after using them a certain amount of time, people are going to replace their solar panels eventually (though it may be 25 years), so there's also going to be business in upgrades.