Network operators who deploy them end up removing them, for the reasons previously described. This isn’t an abstract techno-philosophical stance; I’ve seen this happen repeatedly, after significant network outages which resulted from poor design choices.

Mobile operators need IPv4 address sharing

The way to accomplish this is with NAT64/DNS64 with 464XLAT. This approach is used by some of the largest wireless network operators in the world; if it’s good enough for them, it’s good enough for your customer.

NPTv6 is not a viable alternative for mobile operators because it disrupts end-to-end IPv6 connectivity, which can cause problems with IPSEC and the like; lacks a built-in IPv4 transition mechanism; and has a significant negative impact on the stability and resiliency of the network. NPTv6 shouldn’t exist; and to the degree that it’s even remotely suitable for any network at all, it’s only for small enterprise endpoint networks which exercise a substantial degree of administrative control over the communications of the nodes on said networks.