Paladins, arcane tricksters and eldritch knights are not melee spellcasters , they are warriors(rogues) with spells. Valor bard and war domain cleric can't be full fronliner and pact of the blade warlock makes you very MAD(multiple attributes dependent), so you have to pick additional feats, increase dexterity, rather than charisma, and in result it makes your blade an addition to your eldritch blast, rather than a full focus
I don't understand a distinction between melee spell caster and a warrior with spells. To me it sounds like the same thing, but maybe I'm missing some key difference.
If Valor Bard and War Domain Cleric can't be frontliners, how come Hexblade can be one? Same HD, access to shield, medium armor or heavy (War Cleric). While Hexblade has Shield and Blur, which both of the others lack, Pact Magic limits their uses. War Cleric should be a lot more reliable frontliner than Hexblade.
In PHB you can't have a cake and eat the cake, so yeah, if you want to be doing both melee and casting something needs to be sacrificed. You can focus fully on being melee character with Pact of the Blade, but you won't be as good of the blaster if you invested solely in Charisma. That's fine.
And I didn't understand your logic about providing more options = less variable builds, it's strange
Just look at your list of most popular subclasses, almost all of them are either the most powerful subclasses or the most powerful dips. People will only consider options if they feel that they don't make themselves weaker in the process. Instead of trying to build a paladin around either Strength or Charisma and having two distinct playstyles, most people would dip 1 or 3 levels in Hexblade. Why would you ever try making abjuration frontline wizard when you can just be a bladesinger. Why gimp yourself by picking any subclass other than Gloomstalker when Ranger is already "the worst class in 5e".
And while some will say that they don't powergame on the table, because it ruins fun for other people, in crpgs things look differently.
