MESA program does have a good success but that's only because MESA is a revolving door for pilots, for every 20 that get hired 20 walk out. They will hire anybody. And many fail to realise that yes, you will have a higher seniority but you won't have the hours required to upgrade. Total time needed to upgrade is around 3000 at mesa. That equals to about 3 years of flying FO assuming you started with 300 hours and fly the 75 hour minimum every month. If you had become a CFI you would have the hours and would have been ready for an upgrade in a year or so. Your decision, mesa FOs are the lowest paid in the industry.