Are you confusing DPI with MITM or something? DPI does not require any decryption of payload. Even cheap consumer devices can perform DPI on encrypted traffic

Are you confusing DPI with MITM or something? DPI does not require any decryption of payload. Even cheap consumer devices can perform DPI on encrypted traffic. ISPs absolutely use DPI as a part of standard practice, and have been for decades. It is a basic network traffic management tool.