Uta-napishti
Adventurer
You don't really need any archetypes defined by subclasses etc. You have access to all spells as a mystic. Your spell choice is your archetype. Want to be a life cleric? Pick some healing spells. Want to be a heal wizard? Pick the same spells, but a different backround. Want to be healing warlock? Pick the same spells, but say they come from your patron. Want to be a healing psion... you get the idea...You could make a game that way, and it would work, but I'm not sure it would work better.
If all the spellcaster archetypes (wizard, cleric, druid, sorcerer, bard, warlock, artificer, psion) were lumped into one class, you'd still need sub-archetypes to further differentiate at least some of these (cleric, sorcerer, bard, warlock) and the rest would need either sub-archetypes or new archetypes to cover the whole theme (necromancer, illusionist, mesmer, caster-monk, alchemist)... But I really don't think anyone would call their character a Life Cleric Mystic. They'd say Life Cleric and leave the 'class' out, because it's not necessary to explain what you mean.
In other words: you've added a level of taxonomy. I don't see how that helps any design goal.
Also: with only four possible combinations, I'm not sure multiclassing is worth the effort either. On the other hand, I would like to see spell limits get simplified for most classes, though I think I'd leave spell slots/preparation for wizards (which would then definitely need their own class.)
Bards pick more illusion and enchantment spells, and have a few levels of Expert to pick up inspiration, or performance skills.
Classes are just there to provide a container for some leveling stuff, and some broad association with types of power (Magic, Muscle, Skills), but don't have any roll defining archetypes... everybody does that themselves. If you want to have some example builds to make a Wizard vs a Psion, you can have a list of suggested feats / spells like the Battlemaster Builds part of Taschas.
The idea of Low Class D&D would be to remove specific class and subclass ownership of most abilities, boil down the pool of abilities into 3 big pools, and let folks mix an match to make MORE varied characters rather than less.