The achievement hunting players of Guild Wars 2 have recognized a bit of an issue with the Crimson Assassin achievement. Namely, the fact that actually earning said achievement would take approximately five real-life human years to complete - something that ArenaNet has agreed is an oversight.
According to one Reddit post, the red SAB weapon skins cost a total of 200 Crimson Assassin tokens to get the full set of 16, which is the only way one can get the Crimson Assassin achievement and subsequent backpack reward. Combined with the limited token earnings per day and the fact that Super Adventure Festival is also limited, that adds up to players getting the achievement by 2021, assuming many dailies aren't missed.
One of the devs has responded to the matter, admitting that this achievement isn't quite meant to be so...uh...long-term. "We kept it on standard with the other SAB backpack (SABackpack?) collections, but didn't really consider the sort of pressure this would apply to you," reads the response.
Unfortunately, the devs are unable to make any adjustments to the achievement as of right now while the festival is running for fear of ruining every player's achievement progression. That said, they are looking into a solution that will not require players to get all 16 Crimson Assassin weapon skins to earn the Crimson Assassin achievement.
It's a shame that they're not able to hotfix the problem quite so easily, but at least they're aware that is indeed a problem and are going to work on a solution. Because, seriously, five years or so is a really long time to wait for a backpack skin.
Source: Guild Wars 2 subreddit