You're assuming that not only do we have our current limitations on splitting stacks based on the number of available selling slots (which is clearly true), but also that that was the intended purpose behind that limitation. It's far more likely that the limitation on selling slots is purely about how many listings their Marketboard Database can hold before the time needed to query the database is adversely affected. All those decisions players have to make about how many of their available slots they're willing to dedicate to splitting smaller stacks, those decisions are only necessary as an unfortunate side-effect of a software limitation.
If people could put up larger stacks, while letting sellers take whatever portion they want to buy, that decreases the overall number of MB listings involved in those sales, making it easier on the MB database. So it would be easier on the database (fewer listings), easier on buyers (can buy the amount they want) and easier on sellers (can list more of what they want to sell). Now sellers would eventually want the final few items to sell, so they should be able to offer two prices, letting buyers get a cheaper price if they buy the entire stack, but then it seems everyone would benefit.