i agree, especially during the dacian wars of traian 15-16 legions plus lots of auxiliary troops from the entire empire were in dacia!
the conic helmet type was in the beginning only used by the eastern legions (as you mentioned). also the armour the legionnaires at the monument wear is eastern (lorica plumata or squamata, i can't tell cuz its just a
ing rock
)
this is a dacian/sarmathian example of scale armour:
but the arm protection, the shin guards on BOTH legs (quite uncommon) and the bars reinforcing the conic helmet suggest that the entire armour was modified for dacian warfare!
it's not difficult to notice that the romans designed everything to make the dacian falxes slip off.
for this the conic shape is simply the best design
thats why the dacian nobles themselves used preferably conic helmets.
dacian falx (medium version for combination with shield) and weapons and armour as war booty:
ofcourse the auxilia did not get that much new equipment
they still used the "old" stuff mostly.
the depictions that show roman legionnaires with conic helmets, shin guards, lorica plumata and arm protection are almost all from the ____ war monument. this monument was most probably built by miles themselves that were in service during both dacian wars and before. this means that they had couple of years to modify their armour for dacian warfare. the depictions of the early dacian wars, f.e. in 85/86 show roman soldiers with lorica segmentata and weisenau or even montefortino (
) helmets. funny is, that the lorica segmentata was introduced recently, but had to be removed in dacia, because the falxes (40-50% of dacian warriors used different variations of it) were/are the perfect can openers.
while the moesian legions already had modified armour in traians first dacian war of 101/102, the fresh troops still used their regionally typical equipment. so the dacians still inflicted many casualties and even managed to defend the biggest mountain pass in 101 AD. also they were very successful in guerilla warfare, until more and more tribes defected to the romans.
however, the dacian culture vanished and the romans gained some experience. as always 
here are some early and late depictions of roman armour during the dacian wars:
EDIT: i completely agree with your quotes