Efficiency of cubing with respect to DPM

But yeah... The real "power" comes from NX... The question is, why on earth do some players "invest" 5000 dollars to "achieve" these stats in a free to play game? Just to show off? :/

Precisely. The only plausible reason why people spend so much is because they want to show off their damage and how "pr0" they are. My guess is they don't feel accepted IRL and to counter this they try to be extra good in MS. If people gawk over their damage and become jealous they have successfully done what they set out to accomplish-- to be recognized. It may seem a bit harsh to say, but I can guarantee you this is the case in most situations.

However, there are situations where people just love MS so much they spend thousands of dollars simply because they are rich and want to support Nexon. And also rare situations where people can legitimately make billions of mesos by merching all the time. There probably are people that started from day 1 and played up until now that acquired well over 10b mesos without spending nx at all, but these people are usually business savants who really should be out in the real world using their skill to actually make REAL money and not be playing a game.