With every collectible release related to characters people don't care about (so, the new ones), the value of OT/PT collectibles increases.
Why? Because it's an extremely slow industry. It's months or years of waiting, constantly.
When you see Sideshow release a new PF from Rogue One, or when they inevitably release aa909's favorite, war hero Rose Tico, it pushes back potential interesting releases even more. What does this mean for the secondary market?
Well, no new ROTJ Luke PF will make the old one more valuable. No new cool Sith or Jedi PF will make the old ones more valuable. Etc...
Look at the prices of Master Replicas lightsabers. Not even the ones from the OT, look at the PT ones. Sidious, Yoda, ROTS Anakin... All those hilts that used to be worth barely $300 a few years ago sell for up to triple that nowadays.
Look at the prices for the Mythos line. Madness. I consider myself lucky to own just two.
No matter how successful the movies in terms of box office, an 8 year old kid and his mom aren't going to buy Premium Format statues. The collectors are the hardcore fans, aka the most split demographic of the fanbase when it comes to the new movies. If they have a hard time bonding with the new characters, they will look to the past. That means an increase in value for older collectibles.