There is an element to every MMO that is subscription based, an element that is defining and valiantly defended by all its supporters - the Longevity.
What happened in this Guild Bank duping Fiasco might very well be killer for a large amount of people for these things. To understand this you need to understand all the large scale elements that make a game gain longevity - and the effects of possible large scale duping has on them.
This one is easy. VR players with full legendary sets and ill-gotten gains from duping will dominate and have massive advantage over normal people for until the next expansion to come. This also connects to indirect beneficiarys from this incidents and, even to a lesser extent, to the PvE side of the fiaso - more on that later.
God forbid people can also, and actually, completly invalidate and bypass the main element of the PvP System right now - the siege weapons. How much can you compete with a raid with infinite siege weapons and forward camps at their disposal? You can't. This is especially devastating.
Unfair advantage like this is killer for a competitive system like this.
People with gains from the duping might very well already have the best equipment in the game, and might very well have steamrolled through the veteran and dungeon content by now because they could bypass the balance element of this entirely - by the use of full legendary sets and enchantments when the game clearly wasn't balanced for people to have that. Future content, once released, will be a blitz for these people too - as it will not be and is not balanced on you having to have the very best gear to get there.
Wether it be buying items to deconstruct or duping the materials to craft stuff outright - people can entirely bypass crafting progression with the duping of items and gold.
These people will be extremly hard to track and some of them will even be unknowing of their status, and the effects they cause on the game is just as bad.
Hiding the Evidence
Can be done very easily and on multiple levels. Wether it is spending duped gold, selling duped items, laundering stuff over multiple accounts, hiding it in pending mail, pending auction house retrievals, dead guilds, etc. pp. You name it. There is absolutely no way to track all major consequences emanating from this.
Hell, even the accelerated progression of people due to dropped market prices will have severe negative impacts on the game's future.
The main issue is in a game like TESO, where EVERY ASPECT OF THE GAME is compromised by this, and a bug that has supposedly been PRESENT SINCE CLOSED BETA, the effects of something like this are of an unheard of magnitude and will require drastic steps to remedy.
As solution I can really only see them doing a complete rollback to before the bug existed for everyone, that meaning, since this is apparently a bug since closed
beta, a complete relaunch.
I feel this way because there will be more people affected later down the line realising they cannot compete with duping beneficiaries in ANY aspect of the game then there will be pissed people now.
EDIT: Topic updated with more input from others, thanks people