In the UK at least the fallback for GNSS is intended to be twofold. a network of VOR/DME stations and a network of DME stations. The first is already in place, it's a subset of the original VOR/DME network, and is intended to support RNAV5. The latter is not yet in place, the UK has plenty of DMEs but they are concentrated in the South and not spread evenly across the country, but will support RNAV1. The limitation for most users is going to be suitable equipment in the aircraft. Multi constellation is likely to be much cheaper and more widely available, even my mobile phone can do that, than boxes which do DME/DME.