Li Shenron
Legend
This has a few benefits.
First, people won't be ale to multiclass sorcerer and wizards, which would be tricky to balance, and makes tracking spells awkward.
Not a benefit.
For some people this limitation is a drawback. Furthermore, you will still have the same balancing problem when someone multiclasses between Cleric (or Druid, or Bard) and Sorcerer or another Mage subclass that doesn't use the same casting mechanic.
Second, it's easier to offer support. Feats and new spells can apply equally to the psion as the wizard, making it easier to offer new content. Likewise, if they need to make a new mage variant that subclass isn't starting from scratch.
Not a benefit either.
Feats never directly require to be a specific class, by design, because they want feats to be used for customizing as many characters as possible. Sometimes of course this cannot happen, when a feat is instead designed to provide an improvement to a specific feature that might be available to one class only or a few, but in that case at least the feat is always worded so that if you have another (in a future product, or even your own homebrew) class with the same feature, the feat would automatically work.
There will be feats that apply to all spellcasters, so it doesn't matter if "Sorcerer" is a class of its own or a Mage subclass, those feats will apply. There will be also feats that apply only to spell-point casters, but once again it doesn't matter where is the Sorcerer located.
Ergo, if it doesn't matter, then there is no benefit to have it under Mage.
Third, all classes can use the same iconic spells. Rather than having to hold back the telekinesis spell to make the psion different, or make its version different for the wizard's version, they can both use the same spell. And the pyrokinetic can gain access to fireball.
Fourth, by tying all the dedicated spellcasters together it makes it easier to reflavour magic in general without having to make room for three or four different classes. Customizing magic for your setting is easier.
Again not a benefit either.
If they really want to have one spell list for all of them, that can happen also if they are separate classes, in fact that's what it was in 3e.
If they want to have different spell lists, they'll have different spell lists period, whether they are separate classes or the same.
Ergo, once again it doesn't matter, so lumping them under the same class has no benefit.
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Just think of this in the following terms: designing the class Mage as an empty shell where you can "swap" its whole content, is identical to having separate classes and generically say "they are all Mages", except for multiclassing restrictions and for the fact that (with the current Mage class) the designers are forced to designing all those alternatives to Wizard to conform to the same level-by-level structure (when they get a new level of spells, when they get a tradition benefit, when they get a feat).
In the best case it won't matter, maybe that structure is exactly the best for psions, sorcerers, artificers, warlocks... maybe getting spell levels/tradition benefits/feats at exactly those levels is right for all those classes. But this would be possible also if they were separate classes. OTOH it's possible that it won't be the best for some of them, but then the Mage class structure is effectively a design restriction.