Besides needing some new programming work to identify all open water sources
Unless I'm mistaken, that's already known by the game. The processed splat includes not only the water that is on the normal splat, but I am pretty sure it includes all other water in the game from POI and tiles as well. So that part shouldn't require any extra effort if they wanted to do something with it.
That water purifier for the dew collector feels kinda useless. Because of how many dukes it costs, I never buy it and choose to just boil the murky water.
There needs to be a compelling reason more than just convenience to purchase it rather than boiling water manually.
Consider all of the multitude of QoL updates in games. In many cases, perhaps most, those are things that are just for convenience. Yet people want and like those updates. Convenience is appreciated by a lot of players. It depends on what it is and if it negatively impacts something else, but generally speaking, convenience is enough incentive to use something. At least for a lot of people.
As far as the filter, I always get it not long after making my dew collectors. I buy all 3 mods for each dew collector as soon as I have the dukes, which is usually day 1 or 2 on 2 hour days for the 2-3 dew collectors I'll build. I rarely buy anything, so it isn't like I'm having to choose between the convenience or some other thing. It just makes sense to have the convenience. Sure, I can spend time boiling water... campfires are very cheap and fuel (wood or coal) is easy to get. But I can also just spend a small amount of dukes (they really aren't that expensive, especially if you do any quests) and not have to spend any time at all. In the end, it isn't really any different than buying the mod to speed up the dew collectors or the mod to increase the capacity of them. Both are also just convenience. Dew collectors aren't hard to make, so making extra instead of buying mods isn't difficult. But you'll buy those mods. Those are also just for convenience, so it really just comes down to what you consider a worthwhile cost for the convenience. You're willing to pay for two of the mods that are cheaper, but dukes are more important than convenience for you for the more expensive mod. There's nothing wrong with that, but it doesn't invalidate the value of convenience. You value the convenience less than I do. That's all.
It also depends a lot on whether or not you buy a lot of stuff from the traders. If you are always spending dukes, then you may care about the cost. If you rarely buy anything, then the cost doesn't matter at all.