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Hahaha, somehow I convinced myself that the rationale for the segment register scheme was a long term plan to accommodate virtual memory and memory protection. The idea being that you would only have to validate constraints on access when a segment register was loaded, rather than every single memory access.

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If you read through the PDF Ken links to in another note here, you find this quote starting the "Objectives and Constraints of the 8086" section:

"The processor was to be assembly-language-level-compatible with the 8080 so that existing 8080 software could be reassembled and correctly executed on the 8086. To allow for this, the 8080 register set and instruction set appear as logical subsets of the 8086 registers and instructions."

The segment registers provide a way to address more than a max of 64k, while also maintaining this "assembly-language-level-compatibl[ity]" with existing 8080 programs.

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