Fair points, (I did make a bit of a daft comment about wizz wheels) you guys who have either flown for decades and were trained commercially or military I am sure have far superior DR skills than I or an average PPL could reasonably expect unless they have a serious interest in becoming good in traditional navigation and practice it a lot, some people enjoy that. I personally do not.
In terms of back up the Cirrus has plenty of redundancy in terms of the 2 GNS 430's on independent power supplies, and yes I do carry a further GPS as final back up if it all does go wrong.
With all the risks in flying losing GPS is not one I worry about.