Proton drastically reduced space for paying customers

No, they didn’t. Prior to the change, Lifetime Pass accounts only got 1GB storage in Pass. So in other words before (if you just had Proton Pass Lifetime attached to a free account, you would’ve had:

• Proton Mail: 1GB
• Proton Drive: 5 GB
• Proton Pass: 1GB

Now, you get 10GB shared across all products. In other words, Proton has given you an extra 3GB. Say “thank you, Proton”