This idea that arbitrage can "exploit" anything is non-sense. Are the bids and asks on the internal market not real? Do the orders not represent an actual individual interest to exchange at a particular price? Should traders avoid markets that offer better prices? Is someone forcing them to sell or buy? Why do you think the internal market is slow to react? is 1.5 seconds too long? There is nothing limiting the internal markets from reacting just as quickly as anywhere else.

It should be obvious that that this drag creates a significant amount of artificial sell pressure

What is artificial about it? Do the orders on the book secretly not really want to trade, or secretly want a different price? Why place an order you don't want filled?

In a healthy market, the arbitrage would cause the prices to converge somewhere in the middle.

This isn't true. It has nothing to do with "health"(whatever that means) and everything to do with volume. A large enough order will always bring all other markets to meet it's price, not the bottom or the middle. The internal market price would easily be exhausted and made inconsequential with a large enough order placed elsewhere and vice-a-versa. Where the price meets is simply a reflection of the relative pressures in each market. It has nothing whatsoever to due with "health"

Steem lacks buy pressure. Plain and simple. I don't think we'll see many people crying about arbitrage "artificially" keeping the price high. Even though it's the same mechanism and process in either direction.