Due to economy of scale and some miners having lower verification costs, it creates a race to the bottom of purging all but the most efficient miner (centralization). With increasing block sizes, there would likely be some huge miner that accepts 0.00000001 fee transactions while doing so would cause others to go bankrupt. Or the mega miner could just accept tiny or 0 fee transactions in the short term in order to bankrupt opponents. Hence a minimum transaction fee puts a floor on how low of a race to the bottom tactic you can enact.
Minimum transaction fee is the spam prevention mechanism of Bitcoin, not block size! The miners are unable to create one themselves because it would require collusion and it's also an attack vector they use against each other! That is why I say developers have to create a minimum transaction fee for each block size interval they set in order to scale Bitcoin.
Oh wise central planners. Please pick the best arbitrary economic variables possible... and only with unanimous neckbeard consensus in irc. These miners are hell bent on choking out this golden goose given the chance.
You keep looking but you can't find the woods
While you're hiding in the treesExcept you don't understand it's impossible for Bitcoin to not have central bankers unless it has an unlimited block size and no min transaction fee. Now that we've established Bitcoin already has central bankers (the developers with the miners having a veto override), they should actually use the correct variable in order to block spam (min transaction fee) instead of using the wrong one (block size).
That's like saying it's not possible to have a McLaren F1 without the Ameritech front bumperettes and a 1.8L. It is, and it's worth insisting on.