Let me tell you a story.
I bought a Prius once because I thought I was clever and I was going to get a great car with great gas mileage. I read on the Internet that despite what everyone says about the battery that it is actually very rare for it to die. Well... turns out either that's not true or I got unlucky. My battery died. Not only did it die, but it died an hour and a half way from my house... So during the height of rush our in stop and go traffic my Prius, while it would still run, would take forever to accelerate. I wound up just driving home with my hazard lights on.
When I finally made it home the first thing I did is look it up to see how much it would cost to replace the battery on my car. Well, turns out that when I was trying to replace it (was a few years ago) that the only people authorized to change it was the Toyota dealer. The Toyota dealer would only replace the entire battery though, and that battery was $3,000 to replace. There was no freakin way I was going to pay that.
So after some Internet sleuthing and lots of research I discovered that it is actually possible to replace an individual module in the battery(The battery is actually wired in series of 18 modules). Granted, you would have to replace another one soon if the battery modules weren't balanced with each other. To balance the battery modules though you had to use some funky looking third party charger that wasn't really designed for it and in order to do it in a reasonable amount of time it would cost about $500 because you would have to buy multiple chargers, plus the cost of however many dead modules you had. Then you had to ensure that you could safely remove a 250lbs battery, without shocking yourself with 3,000 volts of electricity. I wound up just replacing the one cell to see how long it would take for the battery to give up on me. That saved my recharging the modules, but the rest of the work I had to do still.
So $34 and 6 months later and I was still driving it. It was only when my wife actually didn't change the oil in the car that it wound up dying (a story for another day).
Which leads me to my point. Toyota had engineered the car so that if there was any battery failure there was no ability for the user to really upgrade it without risking serious injury of death, unless you know... they spend two weeks reading about it and learnings in the ins and outs (which I did). Considering that the battery was ALREADY modular it would have cost them a tiny bit of effort to ensure that users could easily do this themselves. Not only that, but they insisted that the entire battery needed to be replaced because they were also too afraid to allow even their technicians to touch it. All of this could have been prevented if a law like the above existed.
So yeah... I think this is a great idea.