First off, I just want to say that I respect those few people who have posted about their experience with Magic Bands and FP+ during this testing phase - thanks for sharing your experiences.
But, this is a controlled testing phase with very few people participating (relative to the # of guests staying on property and/or visiting the parks). So the lines won't be hugely affected, and the fact that you may have been able to book FPs the day of isn't surprising.
Multiply the testing group X 100 or 1000 (I have no idea how many people are in the testing group right now).
Do you really think that you'll be able to cruise through your vacation and easily book all these things, day of? From the evidence provided by our resident insiders and experts, it doesn't seem likely, but for my own sake (and all of ours), I sure hope that's the case.
But just keep perspective here on the success of your "test" visits, because the likelihood is that it won't resemble your future vacation.
One other point, on the criss-crossing for current FPs. You could very well have to do that on your new and "improved" Next Gen WDW vacation. As a gazillion more Guests (give or take a few
) flood the system, you may not be able to casually book your FPs strategically near each other...even 60 days out. You may have to get FPs for Splash at 11am, then hustle over to Space Mountain for 12pm FPs. But wait, you could only get ADRs for Liberty Tree Tavern at 12:30pm that day, so you better hustle right back over to Liberty Square.
Sounds relaxing, doesn't it? Soooo much easier, booking everything in advance!