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Advertising is advertising. Lots of people say that population-decline is seen in pretty much every on-line multi-player title... perhaps there is starting to be a saturation of game-titles, people are getting busier with their lives, more focus has been allocated towards creating/developing new titles rather than marketing and advertising then further expanding what already exists, the world-population itself may just be declining (over-population being some sort of myth after all), some combination of all of the aforementioned, etc. Mabinogi must have done at least a few things right as there are plenty of people who comment about good times...
You know a game has «Legendary» status when [former] players make such comments...
I had this issue on one of my other computers for a while (within a few years ago). Unfortunately, I do not remember what I did for the fix, but I'm fairly certain that there are some Windows Registry errors to correct. Previous installations may not have completely and fully uninstalled all related files (and registry entries) from the system so you might need to use a reputable complete uninstaller to make sure that all previous instance-components are removed.
Also, if you're using a program like Comodo Internet-Security, your installations might be running in a virtual-box, rather than the actual main part(s) of your computer itself. I eventually did drastic things like re-install the whole entire O/S on some of my systems but if you don't want to take such extreme measures then try to remove all traces of any previous installations before attempting any form of re-installation.
I do not have any specific recommendations (not «free» ones anyway). Actually, GlarySoft has a trusted reputation with most of the Tech-people I know, and so I can at least point to their free Complete Uninstaller. The main program/service that I use in order to keep my system(s) free of errors is with ZookaWare which requires a yearly subscription for full functionality (but it's quite reasonable and inexpensive given that a registration-code works for 5 PCs).
I think you should be able to run Mabinogi after a complete un-installation of previous instances before re-installing.
If that still doesn't work then we'll need to get more information about your system-configurations/etc.