The specifics on Spell design are in the DMG.I can see you're repeating this, but it's meaningless without specifics, and it's clear that even if it's true, it's based on bad design assumptions.
You've totally failed to counter my argument re: skills. I will repeat it, but unless you can counter it, your point fails.
Full casters have the same number of skills as a Fighter - or more (depending on the Full caster), and also have better stats for using skills (INT/WIS/CHA instead of STR/DEX/CON). Even if we ignore spells entirely, full casters are already ahead because of that.
Spell casters have the same number of Skills, but nobody can have all of the Skills. Different party members get their moment to shine based on their Skill choices. Whi h, yes, is DM dependent, but that's the nature of D&D.