A guild who has a monopoly on "good people" would result in several very obvious natural consequences:
- The cost of getting help from anyone in a guild monopolized profession would skyrocket astronomically as the supply is immediately dwindled. If you don't see a massive decrease in supply, your initial assumptions of the large number of bad apples must be called into question.
- The guild's success will be 100% dependent on their abilities to screen individuals. If you put a huge barrier to entry into the more lucrative jobs in the world, you must expect people to teach to the test. Schools will spring up teaching people how to outwit the guild's screening so they can land the good jobs.
- Unless the guild is 100% perfect at screening every single applicant, expect corruption to start to form in the guild because you've literally given them the keys to the kingdom. They are the kingmakers of this world.
- How do you measure candidates aptitude. Every time a "personal nature" test has come forth in the history of mankind, it has failed to properly measure its people. With your example metrics, how would you score an individual who is habitually tardy and sometimes unreliable but, once they're in the room, everyone in the room is almost immediately unified into one positive workforce by their mere presence?
Really, the devil is in the details. How you implement the guild is key. However, its not easy. Remember, some selection process has to be chosen:
- If the selection process is "I want the people in power to have the attributes I like," then you are custom tailoring the process to yourself, and it will likely fail for someone else who values different character attributes.
- If the selection process is "There is an objective set of criteria to distinguish good people from bad," then you run into the enormous challenge of defining good people. This task has eluded humanity for so many millennia that many would argue it literally cannot be dune.
- If the selection process is "The guild leaders subjectively distinguish good people from bad," then your process is identical to that which is used by inner party leadership in countries run by dictators. They also started with a set of subjective ideals for what would make a good leader for their country.
- If the selection process is something in between, the answer gets interesting. People have been writing up in-between solutions to this specific problem for as long as there has been writing. However, it will not be possible for WorldBuilding to weigh in on such a solution until it is fully written and embedded in a particular culture. Typically this is the subject of 1 or more books (Dune, for example, explored some of these answers for 10+ books)