In general, for any software service or system, 100% is not the right reliability target because no user can tell the difference between a system being 100% available and 99.999% available. There are many other systems in the path between user and service (their laptop, their home WiFi, their ISP, the power grid…) and those systems collectively are far less than 99.999% available. Thus, the marginal difference between 99.999% and 100% gets lost in the noise of other unavailability, and the user receives no benefit from the enormous effort required to add that last 0.001% of availability.
