Since you don't have any immediate need for award flights, I'm hesitant to transfer to any particular program. SPG won't lose all value, though as others have said it may lose the 20% transfer bonus. Though you don't use hotel awards, I've enjoyed the few nights & flights awards via SPG over the years and think it provides greater value. If you plan correctly, you can get a cat 3 or 4 SPG hotel and 50,000 miles....and then just keep moving the hotel reservation into the future (when you call you can do this) and you may change hotels, etc...until you need it.
FWIW, I just used a bunch of my AA and Alaska miles on business seats to/from Europe for later this year since I know AA, at least, is going to devalue again. I had forgotten that the Citibank AA credit card, which I've had for ~25 years, includes a 10% rebate on award travel up to 10,000 miles per year so the one way business seat I got nets out at just 45,000 miles (I really wanted first class since they're increasing the rates but I couldn't get any availability).
As an aside, here's a trick that's still possible with Alaska miles. I've got a one way business class flight on AA, using Alaska miles, from London to Los Angeles to Kona (LA to Kona in coach) for later this year with a several month layover in LA. It's essentially a London to Kona flight award, and is 50,000 miles for all. This saved me from buying or redeeming more miles to get to Hawaii later this year.
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