Last time, the Mac platform basically existed in isolation, thus the only problem was that apps for this platform had to be recompiled. This time, the Mac is no longer isolated - millions of developers write client- and server-side applications on Macs that are to be run on mostly x86-based servers, and their toolchain implicitly relies on the architecture being the same on dev and prod machines. That is not to say that it's impossible to change the architecture of the dev machines to something else - it's just a huge additional drawback that was not to be considered at all back then in the PowerPC->x86 transition.
These two facts tend to get downplayed or overlooked pretty frequently when it comes to the "ARM-based MacBooks" discussion, but I consider them fairly substantial and they dampen my enthusiasm for such a transition quite a lot.
