Hrm, this is relatively similar to duplicate-line. I was running 28.2, and duplicate-line was added since that release. Thank you for pointing those functions out.
I do think it's more useful to put point on the second line; I almost always find myself wanting to edit the "new" line, not the old one. Yes, there isn't really a new vs old line, but I write the first thing I want to see first, then want to duplicate it to make a changed second line.
I suspect that although it hasn't been in a released version yet, it's slightly late to have a discussion about changing the behavior to leave point on the second line instead of the first.