I had a border collie/lab mix, and I can attest, they do want to play fetch forever. I had to stop playing fetch with her not because she wanted to stop, but because I had to for her own well being.

She could be dry heaving, slobbering, 1000yd stare... but if I picked up the stick... eyes focused, tensed in excited anticipation, waiting for it to be thrown again

Appenzeller lab mix, can confirm. We always had to end playing because he would have fetched his toy until he died.
He also needed to carry it home by himself (honestly, nobody wanted to touch that thing after a while anyways) because as soon as someone had the toy in their hands the dog would go crazy until it was thrown.

Guess it's the working breeds in the mix?