Our network architecture has the tendency to waste IP addresses. A subnet may have 10 devices but have 256 IPs (e.g. a /24 network like 192.168.0.0 to 192.168.0.255) - that’s 246 wasted addresses. This wastage is kinda unavoidable since we’d need to keep our routing tables from being too fragmented.
With that in mind it is entirely possible for 64-bit addressing space to not be enough, unless we revert to methods like NAT which come with their own disadvantages.
We have already used up about one /11 block of the IPv6 internet. That’s 128-11=117 bits. If we replace the standardized /64 subnets of IPv6 with old /24 subnets typical in IPv4 networks, you get 61 bits. That’s dangerously close to the upper limit of a hypothetical 64-bit IPv5 internet.