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Chess has 1e100 possible games but the more practical metric is that the average game involves 40 black moves and 40 white moves.

Similarly, it’s better to think of the depth of a network’s topology. The size of the tree when full is immaterial, especially when the last 64 bits is intended to be so sparse that random address assignment is viable.

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I missed the edit deadline and wanted to add:

So instead of talking about there being 2^128 addresses, it’s more useful to talk about there being 4k global regions (/12 RIRs), assigning /32s to ISPs in their region.

Each ISP can assign a /48 to each customer site, which can then subdivide into 256 buildings with 256 VLANs each (or some other balance of the these, eg 8 buildings with 64 floors each and 128 VLANs on each floor) with the hosts selecting randomly (or SLAACly) from the final 64 bits.

Some ISPs where customers are in a single dwelling will only give you a /56.