Couple of things:

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-...

From what I could tell, iPads are the only tablets still going strong and getting even better. It’s Google who threw in the towel on tablets.

See the recently revealed iPadOS: https://www.apple.com/ipados/ipados-preview/