The Crimson Binome
Hero
There's the entire concentration mechanic, which limits you to a single buff at a time. There's the explicit rule that you can't spend a spell slot to cast a spell as a standard action in the same turn you spend a spell slot to cast a spell as a bonus action, which effectively limits how quickly you can spend spell slots. Two general rules that apply to all spellcasters make it pretty clear where the designers were trying to go with this.This is the first time I have ever seen anything like this cited as anything like a "fundamental principle". There's certainly nothing explicit or implicit in the 5E rules to indicate that WotC feels this way, and quite a bit to indicate that they don't -- starting with the paladin.
There are even the actual smite spells, which interact with both of those mechanics, and show us how the game could incorporate this type of effect in a consistent manner. The game sets up how you're supposed to spend spell slots, with every rule aligned toward the same goal, and then violates all of that with one class feature. That is bad design. Even ignoring how it negatively affects the game at the table, by letting one character turn into the superstar of every boss fight, it is just plain inconsistent with the guiding principle of spell slot management.