The breakage is generally because of versioned sonames that no longer exist.
You can build backwards-compatible Windows/OSX binaries by shipping everything you need in the isolated package you distribute.
You can do the same thing with Linux as well, but most people prefer one copy of everything, so it all gets built to work together.
calibre has the portable binaries installed to /opt and Sigil was thinking of doing the same...
PPAs exist to make built-to-work-together
newer versions of XXXXX. But they don't work very well when core frameworks used by the system (e.g. Qt5) aren't getting updated and your project requires newer versions of that core framework.
So really, it is Sigil that isn't backwards-compatible with Qt5 5.2.x
