I`m afraid you`re overlooking
I`m afraid you`re overlooking important factors mr. Herron.
You forget that a battery consist of cells. You cannot assume that it will take less time to charge a smaller battery, because then you would have to assume that the smaller battery can handle the same amount of charging power.
In example; the 85kw car has better acceleration, why do you think that is? It`s because the 85kw pack can handle a more massive battery output, most probably because of how the battery is put together.
When "refilling" the 85kw pack will be able to handle more charging power than the smaller batteries the same way that it`s able to give a larger output.
In small scale: would you charge a regular 12V car battery with a 90kw tesla charger? It would take seconds/minutes (not going to do any calculations), but that`s oversimplifying the case.
Bottom line, you cannot say that a 60 (or 45kw battery for that sake) can handle the same charging power as the 85kw battery. Please correct me if Tesla has released documentation of the 60kw being able to charge at the same rate as the 85kw.
I agree that most of the buyers would do fine with the 60kw (and should buy the 60kw), but you are overlooking several important reasons for buying the 85kw pack (less battery wear from charing/use, better driving range after years of battery degrading, better performance +++).