ZA is basically like Zul'Gurub in Vanilla. I believe it's original purpose is to help people who start playing in later phases to gear up to an OK'ish level so they have better chances to join the "main" raids. It would require a lot of work to start raiding from the beginning trough all the previous content and clear them enough times to get at least somewhat geared at this point. Also there are some optional "bis"-items for some classes and individuals that may have not gotten their upgrades from T3.
The Vanilla phase of Onyxia lasted pretty much exactly an year:
- Vanilla was launched 18th of November 2023
- TBC pre-patch came out 2nd of November 2024
- Outland was accessible and TBC officially started on 16th of November 2024
- The first TBC raids came out on 23rd of November 2024.
Based on the schedule TBC content has been released I am assuming it's gonna be pretty much the same with WOTLK. So my guess is that WOTLK pre-patch will be launched around November 02, 2025 and the official transition will be about 2-3 weeks from that. Hope this helps at least a bit! :)

