Is your theory really that closed source software will just stop being developed? I can't find another way to read that, but you can't think Google is just going to give up and shut down, right?
Closed software publicly available. Internal closed software, sure.
But why invest time and money when your features get copied as soon as you publish it?
Look at spaces where ripoffs/copying are already rampant like mobile games. The result is a slop avalanche, rending the whole space commercially unviable for anything other than the most bottom-of-the-barrel dark-pattern shitware laden with scam ads and spyware because users don't care and won't pay.
That may well be the future for most commercial software (and software engineers) as margins collapse and AI tokenomics moves more power from labor to capital.
"AI is going to lead to much less software being written" said no-one ever :)