Most of MM+ is stuff that you will never see, and most of the budget went to wiring the parks, built in the earliest in the mid 1990's, with fiber optic Ethernet links, pervasive Wi-Fi, and then putting that to use. The refurbishment of PoC? That's to put MM+ technology in, among other things, but you won't see it.
Before, when rides went down and required repair, the cast members at the ride must notify WDI, who come over and then must diagnose the issue with tools in situ, before it can be repaired. MM+ means that WDI can know of ride performance problems before they're broken, have parts ready and replace them overnight - all without you knowing. In individual attractions, the individual sound tracks are stored locally and read off of flash storage, but now they sit on redundant systems elsewhere and are streamed in when the local parts fail. Ride availability gets better, ride utilization gets better, and the parks and traffic flow is restored to the way the parks were designed to be used.
The guest interactivity part is such a small portion of what's going on in MM+ as to be almost laughable. 38,000 Wi-Fi enabled door locks, changed while the rooms were mostly occupied, hundreds of thousands of Wi-Fi access points, miles upon miles of fiber optic cable, tens of thousands of Ethernet switches, and innumerable routers and other hardware, not to mention datacenters of servers and other state of the art infrastructure. It literally brought the backbone of WDW from the 1980's into the 21st century, and can now track crowds properly to make the experience overall better for everyone, by providing crowd redirection at will.
And that's all before any of the personalization bits come into play.