From what they are showing Rosetta 2 is a much better technology than Rosetta. Rosetta which came with the first Intel Macs worked by running Power PC apps in an emulator. But it never performed well.
Rosetta 2 seems from limited demos like native code. And that is on pre-release hardware, an iPad CPU in a Mac mini box perhaps, with final hardware with desktop PC power and thermal budgets months away.
Biggest difference from Rosetta 1 is it's not emulating but is re-compiling apps from x86 to ARM, on install from the app store. That was explicitly mentioned, but it can do it at other times, such as on first-run for manually installed apps.
So this is not just a hack designed to keep apps working. It looks like something they could keep in the OS for as long as is needed. It might be especially needed for games which once released are often not updated significantly, but keep selling and being played.