As a long-time EVA flyer, through the years I've observed that their revenue control folks are very slow to react to market demand. The change to SAUA availability no longer being tied to Z class availability arose mainly for two reasons. The first was of course the travel demand post-pandemic. With Chinese airlines no longer dumping capacity on the trans-pacific routes, EVA and most of the other non-Chinese airlines suddenly found themselves with full cabins. They could pretty much sell out 90% of the J cabins from the bigger hubs. So as much as we dislike it, you can't expect a company to give out "free" upgrades when they are able to sell the seat outright.

The other reason is that many long-time EVA flyers were rather displeased with award availability for IM members. Prior to EVA joining Star Alliance, it was very very easy to get a confirmed upgrade, even as a low tier Silver member. If you are able to read Chinese, you can search Taiwanese travel forums and see lots of complaints about loyalty to Infinity Mileagelands not mattering when it came to upgrades. So it seems the change to SAUA availability rewards those who are members of IM, or at least redeeming IM miles.

Now as many have observed, the post-pandemic travel boom is waning so J cabins are no longer full. Theoretically you would think EVA might start making more SAUA seats available to get whatever revenue they can, even if the reimbursement from partner airlines is far less than a straight up revenue ticket. However, this all goes back to my first point about the conservative nature of how EVA operates. It's well known that EVA as a company was modeled very much after a Japanese corporate climate of conservative, careful business practices, with lots of bureaucracy in place.

I think the only thing that may perhaps convince them to open up more partner availability would be when (if?) the Chinese airlines return in force.