why to (not) support "older" compiler versions

[yawniek via Digitalmars-d]

i have seen many PR's and also Forum entries that deal with the 
problem of newer features of the compiler not being able and then 
patching or working around that to support older compiler 
versions.

since it is really easy to keep up with compiler versions and 
even switch
(and not many features are being removed from dmd) what are good 
reasons to keep backward compatiblity?

the latest example i saw was replacing groupBy by a loop to keep 
compatiblity with 2.066.
while not a big thing, it adds up.

since still a lot of useful features do get added into phobos at 
a fairly fast pace,
would it not be better to to keep targeting just the two most 
recent versions and moving
the ecosystem a little bit further.
For people entering the world of D it would be much more 
encouraging to read a lot
of concise code using all the nice features we have instead of 
just lipstick'd C.