Today AA allows you to buy a deeply discounted economy ticket and redeem miles plus a co-payment to upgrade, subject to availability of upgrade eligible inventory in business class. However, no one can say what these carriers' policies will be over the next two years.

NW will not exist because they have agreed to be acquired by Delta, you need to look at DL's upgrade policies, which require purchase of specfic fares to upgrade. I believe United is the same, but I never fly UA.

Will you earn most of your miles through flying or through partner activity, such as credit card? If credit card spending will be an important source of miles ,and if I were just beginning to accumulate miles, I would choose a credit card which allows transfers to several airlines, rather than being locked into one carrier which might dramatically alter its route structure or frequent flyer program, or even shut down.

Options: Starwood Preferred Guest, Diners Club Mastercard, or American Express Membership Rewards. None of these programs can transfer to every airline, you need to look at the carrier(s) you are likely to use and verify that the card is a good fit. None of these programs are desireable for transfers to United Airlines. To accumulate UA miles through credit card spending you are pretty much forced to use their own credit card issued by Chase. For this reason alone I would try to choose another carrier.

Final thought: if your parents live in another country, you may wish to investigate if the airline has a credit card issuing partner in that country which your parents could use to accumulate miles in their own frequent flyer accounts.