Asking specific questions regarding possible military service in future, then not being hired opens the possibility you were in fact discriminated against which is not legal.
USERRA specifcally forbids discrimination in hiring, retention, promotion, benefit—38 U.S.C. 4311
If a possible employer is digging in on future service, this is possibly out of ignorance or military service members allowing the HR department to believe invulnerable. It could be a specific individual who is ignorant of the law. HR should watch and stop this action. If repeated it's because they get away with this behavior. The only reason to ask about potential future service is to NOT hire the individual.
If asked detailed future plans, realize they are looking for a reason not to hire you.
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If leaving at 17 years, yes, very close to active retirement and companies are familiar and have experienced those who return to complete active retirement. In years past, furloughs cause many to return to complete a full active duty and remain until retirement before returning to the airline. Fair enough, business decision by company, must find job....
In today's market, would opine that many carriers would look to the fact your 39 and will likely fly for somebody in a few years anyway. If a good fit, they may just run the 'risk' even if one does finish a career in the military after being hired. A minority of applicants even fit into this demographic. Most out at MSR or after 20.