Why not flag away the mistakes of the past?

[Taylor Hillegeist via Digitalmars-d]

So i've seen on the forum over the years arguments about 
auto-decoding (mostly) and some other things. Things that have 
been considered mistakes, and cannot be corrected because of the 
breaking changes it would create. And I always wonder why not 
make a solution to the tune of a flag that makes things work as 
they used too, and make the new behavior default.

dmd --UseAutoDecoding
That way the breaking change was easily fixable, and the mistakes 
of the past not forever. Is it just the cost of maintenance?