The same situation exists with the Sunshine Coast Hinterland (890/891) and Woodford/Kilcoy (895/896) bus routes that connect to the SEQ network, but have separate ticketing.
I agree it’s not ideal, but I don’t consider that a good reason not to introduce new services into the SEQ network.
To me, the main thing to consider is whether that’s the most appropriate use of public funding. I would rather see it spent on urban bus services tbh, rather than what is basically a tourism service.
If the high cost is an issue for island residents needing to regularly access the mainland, I wouldn’t object to the locals receiving some kind of subsidised fare.