I would say it's not entirely gameguard's fault. Yes, I know that a lot of people got problems with it but I don't think it's the main reason why they decided to shut it down.
I remember what Dianthea said on the forums about the Level Up Gift Boxes. She said the management doesn't know how to fix the problems and therefore just find easy solutions that don't really make any sense (making GBs untradeable/unbankable instead of fixing the actual glitch of mine PQs).
I say the problems stacked up to the point that they didn't want to fix them anymore. They wanted the easy way out and besides they have already made a lot of money off us. For them, the only way to end all of these is to actually shut the server down.