Well, your assumptions may be correct. But on the contrary it may not be correct. Simply for the fact we know nothing what's coming in 3.0 and whether it'll follow the same rhythm or not. It can go both ways really

Pattern indicating that White Mage will always have a spot
If you'd look at past raid content, starting from BCoB to FCoB you'll notice a certain trend that there's more and more AoE healing necessary. With BCoB inexperienced White Mages struggle handling the healing game for Turn 5. It's easily to work around the infirmity debuff if you know how. But it's quite a hindrance for those who are progressing through the content. It's been mentioned before in another topic that double scholars were "a thing" a while back for Turn 5 - or BCoB in general. I admittedly have done that too for Turn 5, simply because it was easier to deal with while being undergeared.

With the introduction of SCoB, the need for White Mages became more apparent. Turn 6 and 8 had significantly more AoE healing required than in BCoB. While progressing you needed every single bit of mitigation for Turn 9 Gigaflare transition.

Now that we're at FCoB, the presence of a white mage is highly desired. Most of the players are still progressing in it.

I mentioned progression a lot. Once the entire party is overgeared anything is solo-healable. Remember the 3-healer meta for Titan hard mode?

Pattern indication that White Mages may not have a spot
One word: Balance. Square-Enix has been tuning classes here and there so any content can be done with any party setup. If a third healer is to be introduced, it should be possible to clear content with any combination of the three. Be it astrologian + scholar, scholar + white mage or white mage + astrologian.

There's also another thing whether they're keeping the armory system or not in 3.0. The new jobs don't have a base class, after all. An indication of base classes being abolished, perhaps? I have no idea!

But don't forget though: The reason why raid parties bring in a White Mage and Scholar combo isn't solely for building limit breaks or for the mind bonus. It's simply because a White Mage is potentially a better main healer and scholar a potentially better supportive healer. If we were to bring two of the same classes who are both a main healer or supportive healer, this will lead to:
1: A shitton of overhealing (white mage + white mage)
2: Really awkward healing game and a lot of stack burns on lustrate because both healers are in Cleric's stance (scholar + scholar)

Simply due to the clash of healing roles in mind and assumption. I think it's safe to assume that this "theory", about main- and supportive healers, will stay even after Astrologian is introduced. After all: Astrologian may have more DPS/buffing potential than Scholars, which pushes scholars towards the main healer role and Scholars taking over the supportive role if they're paired up. Or Astrologians may have a toolkit more suitable for main healing. Who knows, it's really what the playerbase makes out of it.

Fun fact: In Final Fantasy XI Samurai and Ninja were introduced simultaneously. Samurai was designed as a tanking class and Ninja as a DPS class. Interestingly enough the playerbase flipped it around and made the Samurai the DPS class and Ninja the tank class. Astrologian may even be used as a tank in 3.0! (although unlikely)