We are veering way off topic here, so I will give my answer, then recommend you bring up larger, philosophical questions like the above in a separate thread, if you want to discuss further.
Audio as a hobby is just like any other hobby. If all you want to do is dabble a little, get a decent system that's better then your average person's system, and enjoy what you have and move on, an inexpensive system (like the fulla or Modi/Magni) will suffice. As you learn more about the subtleties of the hobby, you begin to want to fill small gaps in your system. You may become better trained to hear little things (like glare, wooliness, sibilance, definition, and more) that you were not able to hear when you first started. You then may want better equipment to produce the sound you prefer. This may lead you to sell your first system and get one "one notch up". You're happy for a while, then you get better at the hobby and you want to move "one notch up" again. That cycle can be very fun and rewarding to some, but also expensive. Some would argue, if you have the funds, just buy something that is 5 notches up from the start, and be done. The challenge with that is you have no way to know the intricate subtleties to help you get to the end game system from the start, so you dabble.
Just like every other hobby, the law of diminishing returns plays a part in this hobby. a $999 amp is not 10 times better than a $99 amp, but it's (likely) better. Even "better" is the wrong term. The $999 amp will sound different, and likely the only way to get that difference is to get that amp. Sometime there are objective measurements like power and distortion. Most times the differences between equipment are too subtle to measure, and the only way you can determine if you like it is to try it.
One cannot determine if a bottle of wine will be good simply by the label alone. One has to try varieties, and different years, and different regions, to develop a baseline for preference. That one can hone in on the flavors one prefers and how to get them. Also, you may want different wine depending on mood, other ingredients in the meal, location, or company. Music is similar.
Is the mangi all you need? Sure. It's the "House Red Wine". If you are happy with that, you save yourself a ton a money. If you have a more refined taste and are willing to order from the wine list, you may be more satisfied, but you will spend some money getting there.
That all said, the HD6xx scales incredibly well. Many would say the HD650 (nee the HD6xx) is an end game headphone. I know lots of people with $10,000 in equipment (or more) to support their HD650. If you have an HD6xx, you may not ever need to get new headphones.
Hope this helps.