You really think this man's stupid enough to do this at this point in the campaign? The convention just took place. This would burn every political bridge he's ever established in his life.
It's not as if he's going to need to ever cross those bridges to nowhere. Romney won't ever be running for another political office.
Several people who are striving to get Trump elected (e.g. Paul Ryan, Mitch McConnell, and others) also worked hard in 2012 for Romney's candidacy. An endorsement for Gary Johnson demonstrates disrespect and ingratitude towards those people, regardless of Romney's retirement.