Microsoft 365 features that help users manage their subscriptions, account settings, and billing information.
Microsoft provided a reason for instituting this change, but if you're like me, you won't find it very satisfying. In Changes to Microsoft 365 storage Microsoft says that sharing storage among all users means that a user otherwise running out of space in their OneDrive can benefit from a user on the same Family or Premium plan who is not using anywhere close to their allocation. That's not totally unreasonable, but it still doesn't explain the decrease from 6TB to 2TB.
As to your second question, Microsoft writes "For existing Microsoft 365 Family and Premium subscribers, this change generally takes effect after the first subscription renewal on or after May 2, 2027 ..." so for now nothing changes.
If you want my personal opinion - which I actually don't recall you asking about - I always thought that 1TB per user was a ridiculous amount of space to give away for free, particularly when other storage providers make good money on selling that amount of storage. I think that may be closer to real answer here.