I wouldn't say its a cock up exactly if the tools prove to be a big step forward. But not being backwards compatible isn't ideal it is makita btw that make the 2x18v gear which are a good step up in power but heavier and a bit numb tbh. I think that's why they've gone to the 40v as its no bigger than 1x18v but more power. For whatever reason they've stayed at 6ah batteries, supposedly 9ah and 12ah cells are a lot more fussy about being charged and discharged correctly but I've not heard of any big issues with Milwaukee batteries? One thing it does do though is make it an easier decision for someone looking to upgrade kit to change brand as like you say its not backwards compatible so you're not tied to already having any chargers or batteries.
That's the issue I had all my other cordless kit is DeWalt either 18v or 54v. The 54v kit is great and being able to use the batteries in the 18v kit is a winner.
My dewalt impact was struggling at times with stuff, not sure if it was age or needing a service but as they haven't updated the model in the 4 plus years I've had mine I wanted something with more power.
If they had done a more powerful 54v version I would have had no doubts about changing brands.
So it's now a case of being in two different battery ecosystems.
There is those battery adaptors but no idea if they are any good