Let's not forget that a 7nm fab has a very limited period of usefulness for the likes of Apple etc. That leading edge is always moving forward and while it might be financially viable for some aspects of the process to be upgraded to the next node, that's not always the case and that's where TSMC's hundreds of other customers join in and the (now old) equipment can be still used for many more years.

Edit: But perhaps with the exclusivity deals, the likes of TSMC are less reliant on spreading the cost over 15+ years than they used to be. To be clear, I was talking about long-term use.