I don't know about making that the default. I never use the jam filter because what it filters out is basically arbitrary. Jams and jam games come in all shapes and sizes, and there tons of unfinished, abandoned, or "non-serious" games that were never submitted to jams. Jam games also may be supported or developed well after the jam period, and jams are even sometimes used to organize collections, bundles, or showcases of existing games. I think this will hurt well-intentioned projects as a whole more than it will help them.
Using myself as an example, four of my five completed projects, including my best and biggest ones, were made for jams; my smallest and most inconsequential one was not. I would hate to have my best work hidden from view by default by a filter that, as you observed, many people may not even notice or bother to turn off. The additional "finished" flag wouldn't help, as then we would just be right back where we started; of course everyone is going to want to flag their work as "finished" for the most visibility. There is currently a flag for release status, but even that is not reliable, as many developers either don't set it, set it improperly, or forget to update it.
Incidentally, I took a quick look at this filter. With it on, I was surprised to see that I still get 370k results back on the most recent tab, vs. 510k without, so it's only removing about 30% of total results. Accordingly, scanning through the first couple of pages, the results aren't dramatically different. Granted this will shift depending on what major jams are going on, but I don't know that it would be much of a boon to non-jam-game visibility overall. Everything sinks like a stone on the most recent tab; even with jam games filtered out, games released less than one day ago are many pages down in the results.