Guess I worded myself poorly. What I meant is "that specific rainbow for 5 rolls" deal won't return.
Guaranteed 6*'s are (probably) here to stay, but if you want to get Maid Cowslip - or any new rainbow - for 250 Gems, you only have two weeks during her event. When she comes back for reissue, she will be available in normal priority gacha - without increased rates for pulling 5/6* in general or a guaranteed rainbow.

And I also screwed up my math in my previous post. Feelsbad.