As I said, there are exceptions, but 6/4 is usually a simple meter. I know it’s not a big deal, but I’ve more or less devoted my entire life to the art of rhythm and seeing people argue with me when I know for a fact that I’m right really irks me. Of course there are instances where 6/4 is felt in groups of three (like how 4/4 can be divided into 3,3,2 but we would never call that the primary subdivision of the time signature), but that’s an example of bending the rules, not the rule itself. As a teacher, it is my job to teach the rules of common practice, and focusing on the exceptions before the rules themselves are learned confuses the lesson. You have to know the rules before you can bend and break them.
I'm not sure I should answer this as you seem so sure of yourself. But I guess I will...
You're talking about popular music. The theoritical framework of popular music is full of mistakes, and it's not written music (for most of it anyway). In classical music (which is the basis for time signature writing, even if pratices have evolved over time, and is written music), 6/4 is a compound time signature. It is the rule. It's the 6/4 in simple meter that is a bending of the rule. In the same way, the number at the bottom of the time signature doesn't designate the feel. It might give this impression in popular music pratice, but it's actually untrue.
If I really need to prove this, here is Brahm's first violin sonata:
Here are the bassons at the beginning of Holst's Uranus, from The Planets (it's in 6/4) :
Here is Scriabin prelude op.11 n° 4 (fun fact, op.11 n° 1 is written in 2/2) :
These are not exceptions, but applications of the rule. You mentioned the "rules of common practice": the music of these composers is the one these rules originated from (I also found examples in Bach, Chopin, etc.).
6/4 is a compound time signature, unless you think Scriabin, Brahms and Holst, were wrong and didn't know what they were doing.
Actually, if you want to be pedantic, 6/4 is a compound duple time signature, because it's two groups of three (9/8 is a compound triple time signature, 2/4 a simple duple time signature, 4/4 a simple quadruple time signature, etc.).
Another example I like is the first movement of Prokofiev's second symphony is written mostly in 3/2, but it switches to 6/4 at several points. A good example :
This is good time signature writing. Also note how Prokofiev wrote 4/4 in the last measure, and not 2/2 like elsewhere in the score.
I'm completely fine with 6/4 considered as a simple triple time signature, but it's a wrong practice that have become an habit in popular music, and should not be considered as the "true" rule. It's not, both from a historical point of view and a theoritical one.