From what I've read Palaestra is just in Sanctuary, not replacing it.
One could see it as Athena's attempts to reform the Saint training system, considering the grueling stuff most of them had to go through against their will before. Just look at what happened to Ikki. I don't necessarily like the idea, but I can at least see from a story perspective why it would happen.
Y'know, I'm curious what the Gold Saints do now. Are there even any Gold Saints other than Seiya? Considering what happened during the Hades Arc, should they even be alive!?