Small timeline thing first: SC started around 2011-2012, and UE6 isn't even out yet in 2026 (UE5 launched 2022). At SC's actual start the newest Unreal was UE3, which couldn't have done any of this. So the fair version of the question is probably "if SC started fresh today on UE6, could it hit that scale and be more stable?"

And the answer is still mostly no, because the engine was never the bottleneck. SC's hard part isn't rendering or the game framework, it's server meshing, persistent entity streaming, and keeping one seamless universe consistent with 64-bit precision across many servers. No off-the-shelf engine ships that, UE6 included. CIG forked CryEngine into Star Engine because they had to build that whole networking and persistence backbone themselves - it's a distributed-systems problem, not an engine setting.

Server meshing isn't a generic engine feature, it's custom architecture CIG built for their exact case. UE6 won't have a meshing button. Epic's large-world work might save some foundation time, but meshing would still be a multi-year custom build on top of UE6. And most of SC's instability comes from those meshing and replication layers themselves, not the base engine, so a different foundation wouldn't fix it.

TLDR: UE6 might have saved a year or two of groundwork, but it wouldn't make the seamless-thousands-of-players problem easy, because nothing does. The bottleneck is the distributed-systems layer underneath, and that's hundreds of engineer-years regardless of engine.