> the largest / longest range model

Nothing has suggested that they are.

You're also neglecting the eventuality where the moment a district needs more buses with a minimum range above some threshold, they need to sell some buses (probably at a significant loss) and buy more new buses. And those new buses need to be compatible with all of the existing systems.

You don't want a fleet of twenty vehicles that need eight different sets of parts. You want a fairly uniform fleet.