It’s an interesting point about effort and memory and something that confused myself. I made a forum post about it because I was under the impression that cards should permanently remain difficult to optimize memory. Any downsides to more and more mature cards?
For me, a benefit of Anki is that it eventually enables ‘fluency’ of topics. If my knowledge of something is more vague and I need a lot of time to recall it, that’s not too far off from using external devices outside of memory.
I believe you need a high retention in order to be able to recall quickly and dependably(fluently). If you with to recognize Kanji in real-world scenarios, you can easily backtrack mentally, use further context, or simply plow on. But for production, if you ‘fail’ in real-world tasks there can be devious miscommunications or the conversation can stall completely. As you said yourself, a lower target retention may not be ideal for becoming conversational.
That being said, how low can you go with target retention and still get those ‘fluency’ benefits? If there is some target retention rate that can be low enough to give you the benefits you mention while working for a variety of different types of tasks and notes, it could be interesting.
I think it would be useful if you expand your experiment to decks that are quite different than vocabulary recognition such as production as mentioned above or something even more different. Many scientific experiments first change settings to pretty extreme values, such as your idea with setting retention to 50%. As long as it doesn’t completely break, if you try such a low retention rate along with very high retention rates and across a variety of types of decks, you will have a much better idea of if and where there is some optimal value.