Acceptible for mid-1st century?

[Crispvs]

I would be inclined to date the manufacture of a helmet such as this to the beginning of Tiberius' reign at the latest (after all, the Besancion Imperial Gallic 'F' was deposited sometime between AD48 and AD52 and was almost certainly made after the Imperial Gallic 'A's and similar helmets and is hardly likely to have been new when deposited). It is certainly true that items of equipment occasionally had very long active lives but it would also be true that any such item's condition would inevitably reflect its age.
Therefore I think it quite possible that an Imperial Gallic 'A' helmet might still have been in service into the AD80s or 90s, but by that time we would expect it to bear the ownership inscriptions of a succession of owners, have a few dents and scratches, show evidence of the tinning having worn off and maybe having been re-applied (possibly more than once) as well as having had one or both cheek guards replaced or repaired.
What I do not think possible would be that an apparently new Imperial Gallic 'A' helmet might be seen in the last quarter of the first century AD. That would be nothing short of a fantasy scenario I am afraid.

As to fashion, you are, I think, only partially correct. Belt styles appear to have changed every twenty five to fifty years. Several styles were in use at any one time but they always seem to fall out of use at the same time. New styles do not seem to be found mixed up with old styles. Other items do not seem to have been subject to fashion in the same way.

When it came to being a drain on the economy, I am not so sure that this was the case during the Principate (although later on it probably was true), due to Augustus having instituted a 4% tax which was calculated to cover the cost of a standing army.

Crispvs