Do not put any serious amount of your investments in company stock. That’s as near an ironclad rule as you can find in investing. And I LIKE Spirit both as a company and an investment, and it is doing well, but not diversifying and instead putting all your retirement eggs in one basket is as stupid as only wiring one AOA sensor to your MCAS when you could have just as easily wired in both. You create a single point failure problem and are a single bird strike away from having to wrestle with a potential catastrophe.