The more software AI can write, the more of a commodity software will become, and the harder the value of software will tank. It's not magic.
Total size of the software industry will still increase.
Today, a car repairshop might have a need for a custom software that will make their operations 20% more efficient. But they don't have nearly enough money to hire a software engineer to build it for them. With AI, it might be worth it for an engineer to actually do it.
Plenty of little examples like that where people/businesses have custom needs for software but the value isn't high enough.