Players aren't earning DEC, Cards are priced in dollars (the same goes with Riftwatchers) so it doesn't really make a big difference. It's mainly in the interest of the company for DEC to restore to peg since they otherwise only earn DEC instead of players buying credits with $$$. They need DEC to restore as they hold a lot of it which the consider as company revenue.
The reason nobody wants to buy riftwatchers I would say is mainly because ther card market has been flooded already like crazy and they don't end up keeping their value because of this. it's 5$ a pack and it doesn't make a huge difference for buyers. If prices in DEC that will in the short run tank SPS even more.
New players have equal access to the packs no matter if they are priced in DEC or SPS.