If the MagicBands are working fine or semi-fine. Why waste more money to change it? Surely that money could be spent elsewhere.
To make more money on bands and those that want to "collect" them, like pins and such. This is all about making more money on them, not less.
Basically, I'm sure their research has shown that only a small percentage of folks ares willing to buy multiples and view them as "collectables" or "fashion" at the current price points. So by separating the guts of the band from the band itself, presumably the replacement bands will be cheaper (I'm guessing sub-$20) which then opens up a whole new segment of folks who would be willing to buy extras.
Consumer behavior is rather decisive on this, by lowering the price below a certain threshold, they will sell exponentially many more bands. There is some "magic" about the $20 price point in consumer minds, which is why I'm pretty sure they will be under that.
To give it a simple example - a kid sees a band they really want in a shop at WDW, even though they already have a band. A lot more parents are going to say "Yes" to the purchase if it's $20 or under and the kid can just swap out the "guts" (or "puck" as they are calling it) out of their existing band, as opposed to spending $40 or whatever on a complete new band and then just putting the existing band away and no longer getting any use out of it.
In theory, this should also reduce quite a bit of headache from CM's and guest services having to continually activate and manage bands for those that do "collect" them, as well - the buyer should be able to just put the puck into the new band themselves and Disney doesn't need to have any part in it, aside from selling you the new band. There will be little to no need for anyone to have more than one "puck", which is the only thing that matters to the computers.
I also expect this to lead to more wearable options - pucks in pins, pucks in necklaces and such - I wouldn't be surprised if we saw long-sleeved shirts/sweatshirts with "puck pockets". (And if they haven't thought of this, I want a cut LOL...)
In any case, I dislike the entire concept of MM+, and the entire thing itself a colossal waste of money, but this is actually a very smart move if they are heck bent on making them an integral part of the WDW experience (and obviously, they are). Given the collector-mentality that so many frequent visitors have, it's a sure-fire revenue stream for them when the puck-less bracelets will likely only cost them a buck or two to make, they have nearly unlimited existing art to print on them, and they can sell them for many times what they cost to make.