Comment by duskdozer

>offering to double the salary in case of success

have you doubled anyone's salary? if not, it can come across as an empty promise you won't fulfill

>sending them to conferences and spending a lot of human time with them

do they want and benefit from these things? or do they distract them from their productive work?

>in most cases, the employer has to eat a shit sandwich.

not really, you were able to fire who you wanted to fire easily. it also seems that you didn't consider other factors for why the employee didn't work out. does your interview process poorly select for people who will do well in the role? are there other possible explanations for low productivity than the employee having a second job?