> IPv4 is pretty simple and good for LANs.

Until the place you're VPNing to happens to use the same RFC1918 network address as your LAN (that is, your LAN is 192.168.10.x and the network on the other side of your work's VPN is also 192.168.10.x). Or either of them use the same RFC1918 network address libvirt is using for its virtual network. Or you want to route between several LANs (for instance, after a company merger) and some of them (but not all) were using the same RFC1918 network addresses.

All of this is avoided by using public addresses for LANs, but address scarcity makes that hard with IPv4 (unless it's a legacy LAN from the 1900s which happens to still use public addresses form the pre-NAT era).

Don't confuse "simple and good" with "flawless" :-)

There are indeed only a few private-reserved IPv4 ranges, and almost everyone prefers to keep things memorable and easy to type; you get a lot of 10.0.0.0/24, 192.168.0.0/24, 192.168.1.0/24 as a result. That, and common household routers tend to default to one of these three /24 subnets. (Hardly anyone seems to remember that 172.16.0.0/12 exists, feel free to use that if it happens to work for you.)

IPv6 does solve this issue in a few major ways, one of which is the greater expectation to rely on globally routable addresses, of which every one of your devices will have at least one such address. There's also fc00::/7 which is fairly equivalent to the IPv4 private ranges, though to avoid conflicts in random VPNs you should generate a random /64 prefix inside of this, otherwise you run the risk of everyone picking fc00::/64 because it's easy to remember/type (I'm guilty of this myself, but the VPNs I've configured just go into a random 172.16.0.0/12 subnet and no v6 assigned. I have the liberty that I currently don't need/use any VPNs that I haven't personally configured, and that may not hold true in the future.)