Probably never, especially with respect to Macs. I am beginning to feel like the cranky contrarian around here for continuing to argue that macOS will never be locked down to the degree iOS is, but I'm going to continue to argue it. You will always be able to develop locally on your Mac, always be able to get to a shell, always be able to install third-party apps without going through the App Store.[1]
But, Xcode Cloud (I nearly typed "Xcloud," hmm) might eventually become what they use as a solution to allow Xcode on the iPad. That would allow them to stick with their "iPads are app consoles" philosophy, even if it might be bending it a little.
[1] With the asterisk that I firmly believe there's a new OS that will replace both iOS and macOS down the road, and I'm not willing to make any strongly-worded predictions about its security model. Yet.