To clear things up, the sign-on bonus was for first mates and first engineers. MSC was so short that they had to sail ships without them, or have seconds fill billets, or promote grossly inexperienced seconds.
The current glut of third mates and third engineers (and seconds) is because the usual attrition from ring knockers using MSC as a brief starter job stopped when finding a follow-on job became difficult or impossible. They just stopped quitting.
The problem now across all ranks and rates is everyone on ships know that when they return from leave and check into the pool to for a ship assignment they will be waiting an awful long time. Knowing this, many mariners simply don't put in for relief or they cancel their existing relief requests. (There is no mandatory rotation or maximum time aboard ship.)
This becomes a bad positive feedback loop: a shortage of timely assignments delays mariners from requesting relief which creates a shortage of available assignments... This is the same as 2008-2009. The feedback loop and the policy changes it created are all the same.
(By 2010 the pendulum swung the other way and people were desperate for reliefs. Deck officers and unlicensed were two to three months overdue. Nothing is forever.)