what will happen after I can no longer use the pixel 1 phone?

[preppystud]

once the pixel 1 phone stops working, does that mean that i have to delete all those photos in order to keep my google drive below 15 gb so i can use other google services, such as gmail, etc?

and how does google know that my pixel 1 stopped working?

 

[B. Diddy]

No, all of the photos you backed up using the Pixel will remain in your Google Photos in the cloud, and won't ever count toward your Google Drive limit.

The unlimited Original Quality Google Photos backups benefit is only from that phone -- it doesn't apply to any other subsequent phone. So once that phone dies, and you start backing up new photos from a newer phone, Google Photos will know it's not a 1st gen Pixel.