Has the cost of building software dropped 90%?

Sure but that's the good of it. Lower labor cost = more productivity. The customer wins in the end because the equivalent product is cheaper or a better product costs the same. Businesses and employees still have to compete against each other so things won't get easier for them in the long term.

Except this is capitalism, so any improvements will go disproportionately to the owners. This narrative of improvements for customers has been repeated for decades and it keeps being wrong.

More stock buybacks and subscriptions!

The customer only wins if the customer is the one using the tools directly, otherwise it leaves all the power in the hands of businesses who's only real goal is maximum profits. And without already possessing domain knowledge to be able to guide, judge, and correct AI along the way, its existence will be of limited use to consumers and business will not feel much pressure to make anything cheaper, it just leaves more margin to funnel to the top.