With rounds of devaluation of mileage plans all around, I was also doubting those plans' real values, and agree with @Jason that not a single airline is necessarily worth 100% of trust, nowadays. Somehow I've been wondering what the *equilibrium* in economics will really look like if the industry is mature enough; flights from point A to B at same fare rates on three competing airlines, deploying similar aircrafts with identical inflight products (from wifi to seats) and services (same flow of greeting and rounds of drinks), namely representing SA, OW and ST? It might be an extreme idea, but we do see some signs today: every week or every day airlines are matching the fares to each other's on competing routes while monopolizing other routes out of their hubs with less competition.
(Hopefully the perfect economic equilibrium won't exist in real world.)
But here in the battle between DL and AS, I think something is different. They are so closely tied to each other that AS will surely be in disadvantage if they don't fight back. It's different from the purpose of FF program to attract flyers. I believe there might be some creative way of gaining benefits with 'friendly' FF programs in contrast to Skypeso's model, but in the SEA market, if not following DL's move, AS is actually ignoring the well-being of their own MVPs, who also deserve a less crowded sky.