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.
Commercial construction sustainably employs a lot of people (and commercial buildings last a minimum of 50 years).
HVAC installation and repair sustainably employs a lot of people.
Roofing sustainably employs a lot of people (and shingle roofs typically last 20-30 years).
I can name lots of other industries just like this. You're just spouting nonsense, or you have unreasonable expectations. If you're expecting solar to employ gigantic swaths of people in this country, you're being unreasonable; you don't expect that of HVAC or shingle roofs or just about any other industry, so why would you expect this of solar?
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.