If they made an access panel they couldn't give them to kids. Choking/battery hazzard
That hasn't stopped every toy made that doesn't come with a pre-installed rechargeable battery from working, which I can comfortably say after Christmas, is still plenty.
If requiring a screwdriver to open is enough for toys kids of all ages play with unattended, surely it would have been enough for a Magic Band.
Disney just either didn't want to pay the cost involved in making this possible with waterproofing, etc. or deal with the added bulk it might have created.
If I had to take a guess, it was all about making them as cheaply as possible, though.
Personally, I think the addition of the smart devices makes sense and is the direction they should be focused on going. That said, this move takes away yet one more thing that helped justify the inflated costs of staying on property.
A more consumer-friendly move would have been to keep providing the bands at no-additional-charge* but make them opt-in optional rather than kind of pushed on guests the way it always was. That way, people would need to take an extra step to get them but they still could for family members where it made sense (kids, older relatives without smart devices, moms who want to go to the pool, anyone who has trouble keeping track of stuff, etc.) and not be expected to pay more for yet something else that was once include when prices were lower but is now a paid add-on to an increasingly nickle-and-dime experience.
They could then have made a long-term plan to phase them out or stop offering them when the bulk of society had zero use for them. Smart phones aren't a 1/1 replacement and despite Apple's success with their watch,
most people still don't have one.
I'd be fine personally with using my phone (I'd even prefer it) but that doesn't work for my eight year old or for a lot of other people. Disney knows they'll be selling these for a good long while and turning a previous cost into a profit.
*I say "no additional cost" because I feel like a lot of people don't understand what the word "Free" actually means when it comes to Disney. When you're paying what you pay to stay in a Disney resort, considering the amenities at that price, about the only thing you can say is free is the air you're breathing. Everything else, you're paying for. Every cut they make should be viewed as a loss in the product that you are not seeing any discount or even slowdown of price increase to compensate for. I guarantee, that's how Disney sees it and is exactly the reason they do what they do with cuts that so many people seem okay with.