I appreciate such a law very much. I find it very irritating how people always want new phones. And I think there are three major reasons to upgrade to a new phone:

1. lifestyle

2. software updates

3. battery capacity

While it is hard to change the first, the other two can be influenced by laws. And while the second is rather complex, the third is quite simple. Since the manufacturers have few incentives to produce phones with replaceable batteries, there are very few options on the market to choose from. Most have other major limitations, like slow CPU/GPU, crappy cameras, or else.

So eliminating one factor of unnecessary waste is absolutely a good idea. I just hope it doesn't backfire in some weird way. And I don't say that replaceable batteries don't come at a cost, they do. But that cost is much lower than many assume and not that easy to measure, because currently, you can compare only apples with oranges.