Compile Compatibility Mode

I can appreciate all you have said. I guess my perspective is, with every line of code, you have a choice to adopt a new library that is supported only on new machines, or to make your project as backward compatible as possible. I never embraced the idea of using the absolute newest compilers and libraries for code development for this very reason. It limits the market to only those with the newest environments.
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Perhaps there's too much code dependent on the new version of QT or the new Mac libraries at this point to roll back. But, going forward, I would caution any open source project from adopting new libraries when those new libraries are not as widely supported as you would like your project to be.
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I'm thinking, why someone uses Open Source MuseScore instead of a commercial product.
1. They are in a country where new computer hardware is scarce.
2. They are a struggling student where finances are tight
3. They don't like their data tied up in closed source projects (often goes back to costs)
4. They are a dabbler or part time musician whose music does not yet support luxury tools.
5. They have or have cobbled together an older audio system because they cannot afford new.
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All of these would seem to influence decisions to keep it working on older hardware as long as is practical.
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Yes, Apple is a special case. Arguably your computer is only as good and safe as your web browser, and since Apple doesn't update the web browser on OSX 10.4 anymore, most people have made the jump to something newer. Still, a few people have, let's say disconnected, older Macs running the heart of their DAW perfectly well, while they use something newer for web browsing. e.g. The VirtualBox / Ubuntu solution mentioned earlier. I guess to compile on 10.4 or 10.6, you might also have to keep one of those machines around taking up space and electricity. I'd think an old 10.4 or 10.6 MacMini with VNC could do the nightly build for that platform.
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The point was, there are ways around it if you are conscious and committed to supporting those less fortunate struggling with some older environments.
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I hope this isn't falling on deaf ears, but rather is providing a perspective that perhaps has not been given the same consideration before.
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I do not want to belabor the point, but

"Advanced Linux users can build by themselves easily though."

I thought I read another post that agreed that forcing users to build themselves might not be the best approach. And, yes, running "make" is easy enough, it's the getting the environment set up that's the pickle. Particularly when it depends upon compilers and libraries that are not in distribution with LTS versions of Ubuntu. I'm still trying to get QT downloaded, which is proving difficult for some reason. It all just further limits your market / user base / distribution potential.
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One more observation. I know everyone wants their new feature, but version 1.3 has been out for a very long time. Perhaps closing releases more often would help push the features out faster, which will help increase adoption, and increasing adoption increases the volunteer base, which increases momentum.