Ignoring most of what OP said, there is an argument for making (attack) cantrips less powerful: balance. Leveled spells (and thus Casters) are very powerful, but are ultimately limited by # of spell slots. However, D&D 5e casters can
also use resource-less cantrips that approach the power of martial characters' basic attacks. True, firebolt does less damage than a longsword---1d10 (5.5) fire vs 1d8+3 or 4 (~8) slashing---but it also ignores the most common enemy resistance: non-magical BPS damage.
Should casters get the best of both worlds? Very Powerful resource-using spells, plus Normal-Powered & unlimited cantrips?
Or should they have Very Powerful Spells but Underpowered cantrips, so that their
average action becomes ~equally as effective as martial characters'?
Guidance won't scale, I guess.... But don't forget about casters with less spell slots. They rely more on scaling cantrips, why limit them further?
Warlocks could be an exception since their class revolves around EB and having few spell slots.
I imagine cantrips were buffed because, as a whole, spells and spell-casting were heavily nerfed relative to prior editions. Or, at least, prior editions with the exception of 4th edition. Spells and spell-casting were made weaker with the reduction of spell slots with few ways of boosting their number, the addition of the concentration mechanic, and allowing re-saves for almost all controlling effects. Certainly their were were buffs to some spells within the context of the system, but as a whole, spell-casting got the nerf bat relative to 3rd edition.
Even so, 5e leveled spells are still more powerful than Martial Attacks (even accounting for things like Sneak Attack, Fighting Maneuvers, and Fighting Styles).
