Yes, it was so. Until you tried to compare apples with oranges and brought .NET with MFC into discussion to say that it's completely new de-facto API and somehow and tried to compare XLib with WinForms. If you want to compare XLib with its counterpart don't tell me about MFC but use GDI+ for comparison. If you want to compare POSIX API use MS POSIX API and/or WinAPI and there is no need to bring .NET into discussion. And it's even better to come up with facts or own experience rather than "possibly broken / probabily incompatible" because that's all I see about so called Windows compatibility issues in this discussion so far. No honestly, all I have from this discussion is some woulda/shoulda/coulda about what could happen with Windows and developers about 5/10/15 years ago and how many books someone should read. I write code for both systems and my IQ doesn't drop by 30 points when I switch from my dev box back to my desktop - at least I hope so .I've mentioned it above: correct me if I'm wrong, but I thought this whole latest exchange of messages started on the topic of comparing native platform programming interfaces: POSIX etc. for Unix vs. Win32 and everything else that was considered as native API for Windows programming.
I can't speak about all applications in the world but let me check on my desktop if you are that interested (still not sure what are you going to prove with that): Far (native) + Firefox (native) + Skype (native) + VLC (native) + VS2008 Express (native) + Cygwin (native) + I download free Kaspersky Antivirus tool every 2-3 weeks (native) + one more IDE(Java) + few games (native but scripts can be in C#/Lua - not sure about this). And your point is ?No - because your assumptions are mostly wrong. Especially "fast forward to now, and most of the code in Windows implementation is probably the same" part - how many pure-Win32 application are there in common use on Windows desktops these days?
Lets stop with examples from hypothetical situations like woulda/shoulda/coulda and return back to facts. I haven't read Petzold and can't speak about all programmers in the world like you do but I've read Stevens and I've read Richter for programming with WinAPI/ Russinovich to learn about internals almost 10 years ago when Win2000 came out - both are valid and very useful these days.And here is another example: in the hypothetical situation I've described above, Unix programmer would probably start learning APIs needed for his work from W. Richard Stevens APUE/UNP books, while Windows programmer would probably use Charles Petzold tome "Programming Windows" (excellent book, back at the time). Still even today, Linux programmer use exactly the same books for learning this stuff. What do you think - how relevant is Petzold book these days, how many Windows programmers had it read (and - don't get started about "Programming Windows Forms", the contents in this one is completely different)?