How would several billion smartphones be able to connect to the Internet without IPv6?
There isn't enough RFC 1918 (or 100.64.0.0/10) space for IPv4-only to be practical: Comcast—not even mobile—went to IPv6 because running their TR-069 management over multiple 10/8 became untenable.
IPv6 is making all sorts of things possible without most people realizing it.
And how would they have gotten first-hop connectivity without IPv6?
Comcast added IPv6 many years ago on their wired ISP side because they ran out of IPv4 for TR-069 management, and they had way fewer subscribers (at least at the time) than many mobile telcos.
And that half of the Internet is also some of the most bandwidth intensive stuff: Youtube, Netflix, Instagram. The CG-NAT hardware costs of streaming would be huge.
No reason you can’t carry IPv4 over any protocol you want. Multi tennant vxlans can carry whatever you want over your base network. Maybe an IPv6 underlay makes sense there, doesn’t really matter