The subscription fee was what enabled them to host these services. From their blog post, they mention spending hundreds of thousands of dollars on infrastructure and software. I expect that the connections and skills involved in running the Rebble web services don't directly translate to creating a hardware product.
That said, I think you are right that Rebble is feeling left out - and that it is hard to figure out exactly how they can fit into Core's vision. But I think there are a couple of primary and immediate issues:
1. Core wants Rebble's data - so clearly there is value here, but Core is framing this debacle like Rebble is irrelevant. Also, I don't know that Google would've ever released PebbleOS if Rebble didn't exist
2. Rebble wants to see the future of Pebble remain open-source or at least compatible with their services, so that if Pebble goes bust again, the community can continue on
That said, I think you are right that Rebble is feeling left out - and that it is hard to figure out exactly how they can fit into Core's vision. But I think there are a couple of primary and immediate issues:
1. Core wants Rebble's data - so clearly there is value here, but Core is framing this debacle like Rebble is irrelevant. Also, I don't know that Google would've ever released PebbleOS if Rebble didn't exist
2. Rebble wants to see the future of Pebble remain open-source or at least compatible with their services, so that if Pebble goes bust again, the community can continue on