Remathilis
Legend
If you can build a character that feels like an old-school monk when you play it then you've lost nothing. People who pitch deal-breaker fits over every detail of the build not being included directly into a stand-alone class aren't going to be happy with anything you do in 5E so they aren't potential customers.
Well, since you brought it up...
Lets say for sake of argument and "monk" as D&D has defined it for more editions than not has the following "essential" traits.
Unarmed combat that delivers multiple attacks and damage comparable to a steel weapon.
Armor Class bonus that grants a monk an unarmored AC equal to armor.
Speed, movement, or Agility/Acrobatic ability.
Resistance to many different types of attacks: poison, mental attacks, disease, etc.
Special Strikes using unarmed attacks that can debilitate, stun, or even kill.
Mystical abilities that grant the monk healing, supernatural senses, or even magical movement.
You can't do that with a THEME. You can't even get close. Why? Because each of those abilities would have to be a single feat, and there is no way you can balance "extra attacks at escalating dice" with something like Reaper or Herbalism.
You can't recreate the feel of an "old school" monk in 6-10 feats, unless you redefine what a "Theme" and a "Feat" is.
Let me introduce you to my friend Advanced Dungeons and Dragons. It has 5 classes - Fighter, Cleric, Magic-User, Thief, and Monk. Druid, Paladin, Ranger, Assassin, Illusionist, are all sub-classes. People played the daylights out of that game. It eventually fell out of favor with the majority of the customer base for other systems that allowed customization of some of the features that AD&D included as class or sub-class based or just generally hand-waived.
As someone else put it; each "subclass" shared a few common traits, but were essentially new classes.
A Paladin had its own XP table, special abilities and restrictions.
A Ranger used d8 HD (and had 2), a different XP table, different # of attacks/round, and different rules for followers along with its special abilities.
Druids capped at 14th level, had special rules for advancment, used a unique XP table, weren't restricted to blunt weapons, couldn't turn undead, and had their own special abilities. Oh, and they're spell lists were nothing alike.
An Illusionist had more spells per level (but capped at 7th, not 9th like an M=u), a unique XP table, and a unique spell list.
An Assassin had better weapons, different XP, a finite level cap (even for humans), worse core thief skills, and unique special abilities than a Thief.
And that doesn't EVEN begin to discuss Unearthed Arcana. I might've conceded the argument a bit more if you had chosen 2e as your example; they did a better job of combining base classes with sub-classes (such as combining the M-U and Illusionist spell lists for all wizards) but 1e subclasses are very unique. "Subclasses" just meant the classes could stand in for the base class. A ranger was equal to a fighter for purposes of combat. An illusionist resembled a magic-user in that they were squishy wizard-types.
And as for "falling out of favor"? Need I remind you that 2nd, 3rd, and 4th edition are ALL class-based games and 3e and 4e have more classes than 1e and 2e put together.
Don't think your desire for 1d4 generic classes is a majority view of most D&D players, or we'd all be playing GURPS right now...