Your HoldPkgVersion idea could have consequences when doing upgrading. If you are holding a package foo and then bar updates to requires a newer version of foo, then you would have to hold back both foo and bar. Its like doing a partial upgrade which is not supported.
As for php53, if such a package can co-exist with the latest version then I imagine you would just need to find a TU willing to take care of it in Community.
Maintaining multiple versions of the same package (foo) is not going to happen unless the dev for package foo also uses foo(n-1). python2 and python3 is quite unique as its so widely used and its going to take years for python software to switch over.
If a widely used project which one of the devs uses depends on an older version of the library, the older version will be kept. Simple pragmatism, unless the person who compiles the package uses it, its not going to get tested and hence shouldn't be in the binary repos.
Allan-Volunteer on the (topic being discussed) mailn lists. You never get the people who matters attention on the forums.
jasonwryan-Installing Arch is a measure of your literacy. Maintaining Arch is a measure of your diligence. Contributing to Arch is a measure of your competence.
Griemak-Bleeding edge, not bleeding flat. Edge denotes falls will occur from time to time. Bring your own parachute.