I'm not sure that people would have a problem with that, if it was introduced that way from the beginning haha. "We're going to have two jobs for SB", info we already have. "One of them is going to be an advanced job with some side content everyone can take part in even if they don't spend time to main it. Advanced jobs are going to bring you additional ways to play the game and spice up the general progression (blue mage learns abilities but levels faster)."
Community: "oh that's different, I wanted some more side content.". Or "Finally I wanted to main blue mage for so long, and new side content? Noice!".
At this point though yeah I think people would be annoyed, if SE said "We're canceling a job for blue mage". Because now you're taking something away (taking away is almost always a negative reaction lol); however, if we started out with the above then I don't think the community would be annoyed - sincerely I don't. To be fair I didn't share a time line with you in the post but I believe they could, maybe, get three jobs out (like they've done before) by starting these advanced jobs in limited state (like now) and updating them into an advanced state over patches (but also telling players their plan lol). Such that you get two at the launch of an expansion and one put between two expansions with explicit info to the community that the limited job becomes advanced, that they're using this as an opportunity to develop new content and get us more jobs than they could normally.
Also worth noting that while the skills have some different effects this job will utilize many of the assets already developed such that Blue Mage becoming an advanced job is more cost effective now than making a new job entirely (many assets are already made, and in game - lore, art, code).
6.5 content begins to lull, beastmaster is released.
7.X (earlier is better, but I'm assuming 7.3-7.5), beastmaster is advanced job. Side beastmaster content is continued to be developed as community enjoys it and SE has ideas.