Modal can enhance UX. I've given examples in my post including an answer to your "Cheaper for whom?"
I don't think it's a good argument, you could also just use your own argument against you: Pages are only easy for developers but worse for users since you don't have to polish the UX in the way that a modal lets you. It's for developers that don't want to have to think about UX optimizations and defer everything to out of the box browser experiences. Which, granted, might be the best choice for everyone who doesn't want to think about UX.
But I find that to be an empty claim.
> These days almost everything is a single page application so instead of opening a modal window just expand the button that would have opened the window inline instead.
That is often a good choice. However, sometimes a modal makes more sense for an all-or-nothing task, or when the user is already in one of those "expando" forms, or a number of other occasions worth pondering for your app instead of dismissing modals without discretion.
Knowing when a modal does and does not make sense is part of a good intuition about UX.