So, the issue with separating the My Posts button out now is that there's nowhere to put it.
I'll bow to your knowledge and experience about what's feasible, but as I pointed out in my OP, it looks like there's space in the subforum header bar, to the right of "About forum", on a desktop with Skinner Mode engaged anyway - in fact, it almost looks as if space has been deliberately left for an extra button!
Failing that, there's space in the lefthand side column. I dont think it would be obtrusive to add a button at the top of that column - in fact, since that space currently carries details about our own account (which wasn't displayed in forum view in DU3, and I really wouldn't miss in DU4), it would arguably be in context sitting underneath name, star member status, post count and joined date, and I think it could work well.
It looks like this is a change quite a few people would like to see from the responses on the couple of threads today. I understand your point about time and familiarity, but no amount of familiarity is going to get around the fact that the old system of showing post replies was extremely convenient and user-friendly, and the new system just isn't. Since posts and replies are obviously absolutely integral to a forum, I'd urge you not to dismiss this as just another among the large number of gripes you're no doubt fielding at the moment. A few minor tweaks like this could help make people feel more at home, and possibly more receptive to the other changes.