There is very little hardware that would actually be ipv6 incompatible. We're talking network equipment from 15+ years ago, which is also obsolete because it's 1Gbps at 10x the power usage of a 10gbps switch.

Almost every network in existence runs on layers of tunnelling, so you can run arbitrary protocols over fixed hardware. We tunnel IP over Ethernet and then we don't have to replace our switches to use new IP versions or features. Most clouds use VXLAN. Many ISPs actually tunnel your IPv4 traffic over a purely-IPv6-only network, to a specific device whose job is to deal with legacy IPv4. The reverse is also possible if you have a network that can only handle IPv4.

If they set the deadline in 10 years, there would be (smaller) mountains generated in that period anyway.