Emacs has a loyal user base, and changing to having cua-mode as the default would likely anger most, if not all of them. I highly doubt that the number of long-term users gained by the change would be even remotely significant.
This sums up well the gigantic hurdles Emacs faces as it attempts to stay relevant over the next 20 years. We're talking about a single keyboard shortcut to switch back to legacy mode.
It's only when you stop looking at growing absolute user numbers and instead get into percentage, is when you reach a conclusion like Emacs is struggling to stay relevant. That's like saying Lamborghini is struggling to stay relevant because of Toyota or something.
My point was that making the default cua-mode won't lead to a significant number of new long-term users, so it isn't worth the hassle it would create for people who are already using it.
Also, emacs is not struggling to stay relevant. I fully expect that it will appeal to an audience of users similar to the one it appeals to today for decades to come.