Not so much planned obsolescence, but definitely cost-cutting. i am not surprised with Dyson tbh, and it stinks. but remember this guy has never invented anything new, just adapted.
I believe this is 100% intentional on their part. On the V6 (SV09) BMS boards, they even have the traces and pads for the balancing resistors, they just aren't populated. All they needed was 5x 220 Ohm 1/10th watt resistors which would have cost them in total about one cent.
If this was truly about cost, I feel like they wouldn't have implemented a fancy BMS IC with built-in balancing MOSFETs in the first place. They probably could have used a BMS IC without balancing FETs that also didn't require a separate microcontroller and saved a ton more cost by removing the uC.
On the later V7 (SV11) BMS board, they didn't even include the pads and traces for the balancing resistors and clearly miswired the BMS IC so the internal FETs were shorted out and could never be used. They did add a reed switch to the SV11 BMS to detect when the battery is removed from the vacuum, which I bet cost a whole lot more than one cent.