There have been many many requests to include a soft switch for ATTI mode on the Mavic, but no indication that there ever will be one. But, if you never switch on ATTI mode on your Phantom, then you won't miss having a switch on the Mavic. Honestly, I don't know why anyone would want to manually use ATTI mode unless you are trying to qualify for a commercial license.
The Mavic will switch between GPS and OPTI on its own, depending on GPS strength, but you wouldn't know it without the controller informing you, it's so seamless. Contrary to what many might tell you, the Mavic is not likely to go "haywire" when switching between modes on its own; it is rock-steady no matter which P-mode it is in. If you find yourself in a situation where the Mavic goes to ATTI mode, like say, 50m up and a sudden loss of GPS (which is very unlikely) your piloting skills would be tested in getting it back home on a windy day. And the controller always announces when it enters ATTI mode, so you're aware you're losing automatic positioning.