Comment # 8 on bug 960738 from Jan Engelhardt

>I think its not an unreasonable request to have a distribution be backward compatible to some extend to previous releases.

Though possible, problems with having two different library versions in the
program address space (can happen if you link to an "old" libX and a
"still-fresh" libY that is linked against a "new" libX) are well described.

It gets worse. On Debian 9, libraries even retained their SONAME, but an "old"
program (D7/D8) passing a struct{std::string} to libX which passes the string
to libY which plays with it, crashes (because std::string changed).

That latter may be an isolated case, but it shows just how bad things can turn.