There is a valid reason ( or many) as to why the newest gadgets aren't always used. I will list some here, many of which you have already read above.
1. The 2nd rate stuff works well against a 5th rate military.
2. We payed for it, no sense wasting it by letting the expiration date go by wihtout a trigger being pulled. (I always hated the Navy throwing expired small arms ammo overboard instead of letting us shoot it up!)
3. You don't want any potential adversary that might have a decent military knowing what you have so they can learn how to defeat/minimize it in advance of it cleaning their clock.
4. You would hate for something to happen and your enemy capture a specimen thereby cutting 20 years off their research....
5. The older stuff is usually cheaper in dollars per copy..
PS: I wanted Crusader and Iowa myself... Along with the Mk 71 MCLWG....
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``There's a fine line between courage and foolishness. Too bad it's not a fence. ''
- (Unknown)