In any case, not quite. Flight capabilities of
galliforms for example are not too well-developed, but
also very rarely lost (not quite as rarely as in
passeriforms, but far more rarely than in the usually
well-flying Anseriformes).
Actually, galliforms have extremely well developed flight abilities,
but they are specialized for burst launching. The tradeoff is that
galliforms have reduced sustained flight ability (low endurance muscle,
relatively high drag at moderate speeds). Galliforms also develop the
flight apparatus early in ontogeny. They hatch with a well developed
pectoral girdle (compared to other birds), and it grows and fuses early
in life compared to other clades. This would appear to be a major
mechanic behind the low incidence of flight loss in galliforms. In
addition, galliforms generally have relatively low wing loadings, at
least at small to medium sizes (galliforms have a fairly steep loading
scaling relationship, because the m. pectoralis major and m.
supracoracoideus mass increases very rapidly with increased body size
in order to maintain the burst launch performance).
By contrast, Anseriforms tend to mature the flight apparatus a bit
later. Perhaps even more importantly, anseriforms have gracile humeri,
relatively small sternal keels (which reduces total bone investment a
bit), and high wing loadings. These are all associated with rapid,
high endurance flight powered by mostly aerobic muscle. Such
long-distance flight creates an impression of "better" flight
abilities, but the morphology and structural requirements associated
with high endurance, highly loaded flight in open environments are
somewhat less steep than for burst flight. Endurance flight morphology
actually predisposes endurance flyers to flight loss under the proper
conditions.
One related sidenote is that while galliforms (and columbiforms, which
also have burst takeoff adaptations) are popular avian flight models,
they are poor models for the "minimum" flight requirements. This has
been a problem for the conclusions of a number of studies trying to
reconstruct avian flight evolution.