Main issue with RNAV is that GPS is just not that precise. WAAS helps a bit but it's not as good as having advanced local augmentation or things like RTK, and those are also non-trivial to deploy (not to mention there's no common standard for ground augmentation for aircaft to use).
So ILS or MLS had the benefit of being able to provide better precision without things like realtime ionosphere monitoring and correction. TLS let's you use all sorts of augmentation mechanisms without modifying the aircraft too.
So ILS or MLS had the benefit of being able to provide better precision without things like realtime ionosphere monitoring and correction. TLS let's you use all sorts of augmentation mechanisms without modifying the aircraft too.