Andthey’re not going to now, and they let Republicans off with almost no pain. Not a winning strategy.

Doesn't mean the alternative is a winning strategy, either.

"In order to make sure you understand that the GOP is cutting ACA subsidies, we're going to make sure you don't have essential governmental services" is certainly a way of applying the 2x4 to the jackass. But it's not one that endears to the jackass the person applying the 2x4. Or if you don't like that metaphor, it doesn't endear the person who stands by and watches the jackass get beaten without stopping it to the jackass.

The Democrats didn't have a potent weapon. Had the public blamed the GOP for the shutdown significantly more than the Democrats, then they might have. But that didn't happen. The public only slightly favored the Democrats over the GOP. Which is pretty impressive, given prior precedent - but not enough for the Democrats to escape the consequences of really consequential losses of essential government services.

Once you got past the only elections that are happening for a year, you enter a serious problem of diminishing returns. You've already made the political point (people are aware of the ACA subsidy expiration) you wanted to make, without hurting too many people too badly other than federal workers. But now you're in the phase where a ton of people are going to start getting hurt very badly, and you don't have that much more incremental possibility of political gain.

If the end game isn't to get the GOP to agree to something more in the CR, then the Democrats had already gotten most of what they wanted with only a modest amount of hurting people. I don't think it's a winning strategy to signal that the Democrats are very willing to let people get hurt - the "Yes, sorry about that" position - in order to get the same amount of nothing, just a few weeks later.