It's my understanding that 6nm is a variant of 7nm and isn't compatible with 5nm which is totally different, ie it would cost a lot more to port from 5nm to 6nm than what slight saving you see.
Great intuition, although 6nm is the 7nm node with 5nm rules(that is the striking feature, that it can be backported to DUV!). It foregoes EUV, but that is what you get for cheap process node updates.
Just imagine TSMC can differentiate 2 DUV nodes(enabling clients one migration without the redesign hassle and without any over investment) in the process pipeline of 1 EUV. I mean - what a company! It is huge...