I'll start to believe in some kind of serious troubles when all legacy realms are down for 1 week and above.
For now servers are up and working fine, the only component is not working is the part that responsible for the game creation, and also only on Europe. Legacy, especially after the mass exodus to d2r literally can be hosted on an average home PC, so talking about resource savings for d4 is funny.
What is not funny is the ignorance of Blizzard staff, who is responsible for the correct server work. It's like the Arreat Summit, which was working slow as turtle for many years, crashing the page every 2 minute, and then became en error page for the several weeks. But now it's forking fast and smooth. So they can do it right, when they want to.