You keep fractions of coins; they just aren't shown. So if you have 50 coins in the bank, you will get 0.5 coins each day but you will only see the total change every second day when you've gained a full coin.
I'm not really a fan of Koikatsu although I won't automatically skip a game because of it. I like both Daz and Honey Select although sometimes I see stuff made in Daz that just looks awful especially with facial expressions. From what I've seen, HS always looks decent but rarely spectacular...
Even if it isn't that difficult (and that's subjective), it still does take some time and effort. That has to be weighed against what actual advantages you would gain. If a dev doesn't feel they would gain enough from changing engines, then why should they waste their time.
Often it is because of familiarity. If you already know how to use a specific engine, it's generally easier to keep using it rather than learn a whole new engine.
Something I actually found in real porn before I ever got into AVNs is watersports. I find it very hot to watch but not something I'd actually want to do.
I had Steam recommend College Bound to me. I think it was because the first couple chapters were free to play so it had the same tag as some free idle games I had played. Of course, once I tried it, Steam started recommending more adult games and here I am a couple years later.
Paradise Found is a real porn game I really enjoyed. I liked the game mechanic of managing your Memory Health so that your amnesia doesn't come back while you gradually corrupt your family and neighbors. It is sandbox which I know some people hate but the map tells you where there are new...