One-letter binaries while convenient are prone to some issues (lack of descriptiveness, risk of overwriting someone’s alias, poor discoverability & searchability), instead, personally, I usually prefer to use one-letter aliases if there is a command I use very frequently (e.g. alias c=cargo).
Additionally, d is non-searchable in google by itself, whereas dfxenv or dfxvm can be.
Optionally, I would go for dfx if it not must be another tool?
At SDK team we’ve reached the conclusion that it must be a separate tool. Bummer - I know; it was one of the first things we’ve looked into.
Update: we have decided to go with dfxvm as the name for the DFX Version Manager. Thank you all for helping with the naming! The repo will be found here once it’s open sourced. That should happen soonish.