Is there an argument to be made about allowing spam on the chain because there's limited demand for monastery settlement?

There's an argument but it's probably not a good one. If demand for blockspace doesn't pick up before we've had a few more halving and/or the price appreciation slows down significantly, we could have a situation where miners can't afford to sustain the high levels of hash power they have invested so much into producing. They might look for ways to increase fees by facilitating more data storage applications, but it really doesn't seem like there's much demand so it probably wouldn't do much. Thankfully the difficulty adjustment will keep the mining industry from collapsing completely, but we could see a washing out of less efficient operations and some consolidation and centralization as a result. Not sure if any of that is likely to happen or not. I think fees will pick up the slack for lower coinbase rewards by then.

IMHO, Fees are not low or high; they always represent the fair value of using the network at a particular time. The filter issue is not related.