ADS-B over Mode S is mandatory in large parts of europe allready, however i do not know how that will help with infrastructure problems in the future.
I do agree that working fallback plans have to be developed and in fact the regulators worldwide should try to get something working on the road which will be used worldwide and not the current localized thing we see. In europe it seems we are going the way of DME/DME as backup coverage, i do not know how it is handled in other parts of the world though.