True, it makes sense conceptually but in practice it would be messier to implement than I feel comfortable right now: as you said, it would be either incomplete since you can’t push through other non-empty lines or if it would create new columns then the cursor would need to go to those new delay columns to continue stepping as well which is not possible with the current tool API. If this changes I might reconsider adding this but I’d rather have something simpler but more general than something that can do many things incompletely (the latter already describes my life, I seek refuge from that here 
).
The other thing I don’t want to deal with is the case of delaying multiple notes at once where different lines get moved at different steps going out of the selection area, or new colums getting added that should be reversible via stepping in the opposite direction etc. It’d get dirty too fast, both for me and for the user, so I think it is better to omit this functionality or make a separate tool just for this, which would be easier to maintain as well.
Currently I use Super Nudge to move lines btw, although it also has the limitations you mention.
That being said my goal was never to make your tool obsolete nor to reproduce every feature it has, if you find anything useful in my code feel free to use it or build on top!