"And what happens when the worthless fiat soon-to-be-toilet-paper currency collapses..."
-Are you expecting hyperinflation? Gilts are in negative yield so its highly unlikely. From my reading of the above Microstrategy are buying Bitcoin as a commodity to park their wealth at a higher return rather than for actual use currency.
What will we be paying our taxes with then?
-If you are talking worldwide collapse then I cant see a commodity like bitcoin getting you out a hole. Money requires legal tender status and a tax paying polity as guarantor. Bitcoin has neither.