For most of us, it's just a matter of a different intermediate stop. Singapore, Bangkok, Dubai, or Perth.
For several reasons, Perth is not the optimum break point.
* it's unbalanced.
* It probably won't involve FLounge access. For OWE Y pax, having that Syd/Melb and DBX FLounge access almost makes Y bearable.
* The intermediate stop will involve immigration and security checks, either at the inconvenient existing PER international terminal, or via new facilities in the domestic terminal. Either way, it's more hassle and expense to be levied on passengers. I can see Customs jacking up at having to staff two facilities, and if there are any staff shortages, things will go pear-shaped fast.
* With the EK partnership, DBX is a great entry to Europe - or nearby Middle East destinations. If I'm going to Lisbon or Tehran, being able to avoid LHR and an extra flight is a bonus.
The long sector is a huge problem in Y. It's not unbearable, but for tall or large passengers, those stuck without aisle access, or those seated beside "difficult" travellers, such as kids or drunks, it's going to be torture after a while. Especially if the flight is full.
For PE or J, it's not a problem, but if I'm travelling Y, I'm going to avoid this sector.
If Y had fewer seats than a regular configuration, giving more room and more aisle access, then it might be okay. Consider the difference between the upper and lower Y cabins on the A388. If the flight was never fully sold, say an 80% loading, that would make a difference.
It's up to Qantas to work things out, but I think they'd go for selling it as a shorter, more convenient way to London, and for Perth folk, that would work. There's a lot of Perth residents with strong ties to the UK. For those of us east of Adelaide, the overall flight time would be about the same, and not so convenient.