I suppose I can understand the backwards compatibility angle. However at least personally I'm of the view that anything accessing the network during a build should be killed with fire. I draw a hard line against using dependencies that won't build in a network isolated environment.
Yeah, I think forbidding network access within build systems is would be a great default to employ.
(I wouldn’t be surprised to learn that a large number of packages in Python do in fact have legitimate network build-time dependencies. But it would be great to actually be able to quantify this so the situation could be improved.)