Earlier quoted context omitted.
As people use desktops less and less, they care less and less about what runs on them.
That narrative of the post-PC world collapsed when the sales of tablets like iPads collapsed.
1) Tablets aren't the only "post-PC" platform. Most of the leisure computing is done on smartphones when it would have been done on a laptop or desktop 5-10 years ago.
2) If we're considering tablets to be PC replacements, we should expect their sales trends to be along the lines of PC sales. People keep PCs a lot longer than smartphones, so we should expect people to keep their iPads a lot longer than their phones too.
3) Tablet sales have absolutely not collapsed [1]
[1] https://www.statista.com/statistics/272595/global-shipments-...