I don't think that the Shadow system is "THE solution" to the problem or, really, the problems as there are a series of related potential problems involved. I was, however, curious about what someone like
@Remathilis would think about such a solution of the Shadow system, which has both basic classes but also hyper-specialized ones.
IMHO, there is no "THE solution." I enjoy SotWW, but I also enjoyed True20, where there were just three classes (Warrior, Expert, Adept) that you could multiclass between for feats and talents to build your character concept. I enjoy WWN, where you have three classes (Warrior, Expert, Mage), but there is both a hybrid class and you can switch out your class for a more focused one (e.g., Elementalist, Healer, Necromancer, etc.). It really depends on what the game is going for. And you know that I would definitely be interested in your idea, albeit for a different game than D&D probably, of a more MOBA or WC3 style game where you have a lot of character options, you jump into the class fantasy with abilities pretty quickly, but you can also get loot to customize or vary things up as you play.
In my humble and controversial opinion, the problem with classes for a game like D&D stems from presenting itself as the big-tent entry point for a generic fantasy adventure roleplaying games. This comes with a number of class issues as it is balancing tradition with evolution, having too many with having enough, and the generic with the hyper-specific. I don't think that D&D is doing that great a job at this, if I am being honest, but I also recognize that changing this would be an uphill battle that would leave a fanbase that is quickly angered by change upset.
To me, for example, the question of where do you get your magic from - bookish studying, freaky accident, a magical patron, music, gods, etc. - is not all that interesting. This would not be my basis for creating classes, especially if I was going to slap on the same basic spell-casting system on everyone.
What I think that a game like Magic: The Gathering* or a lot of MMOs, ARPGs, or other video games get right is that they often understand that player choice for class is often about playstyle and aesthetics. IMHO, a lot of those common playstyles and aesthetics map pretty well, though not always perfectly, to the five colors of MtG, which are designed with both in mind. Want to be the dark edge lord caster? Black. Want to be the support-focused white mage? White. Want to be a nature mage? Green. Want to be the caster that just wants to makes things go boom and roll damage dice? Red. Want to be the smarty-pants with counters and brains? Blue. These are common enough playstyles/aesthetics that I regularly got new players in D&D asking for some variation of "What should I play if I want X?" with X being one of the above.
While one could vaguely map these things to some of the D&D classes, I think that the similarity is mostly superficial with little mechanical support to back these things up and sometimes the aesthetics are a bit schizophrenic (e.g., Warlock). I'm not saying that D&D should just do MtG either. I'm just pointing to it because I think that it does a better job of designing around playstyle and aesthetics.
* For the record, I think that it's almost criminal that the WotC has owned D&D for like 25 years but not had the D&D crew design a TTRPG from the ground up for Magic: The Gathering.