Maintenance also costs money.
As far as those statements about 5.8mm rounds go, the 5.8mm standard round has about comparable performance to the American M61 round. It is somewhat superior at short ranges; 10mm penetration versus hardened steel compared to 7mm penetration, but tapers off against the heavier round at long distances due to lighter projectile weight resulting in increased energy loss.
Looking up NIJ Class IV, M61 is included in the NIJ Class IV category, but the newest tungsten rounds would be class V.
In this case, the GJB 4300-2002 standard is approximately equal to NIJ and GOST armor rating systems.
The standard round for NIJ Class IV classification, I think, is the M2 AP round, which has a penetration of over 20 mm of steel at a range of 90 m.
On page 66 of this
In the same document, if you read page 51-52 you can find the condition for care. Military training condition will be a lot more rigorous than police training. A police officer is not expect to tread through water, mud, hills, pouring rain, fighting/training in over 30 degree or below 0 etc while carrying 50-80 lbs of gear etc. All these conditions will accelerate the deterioration of body armor.
True, but the life of that vest should still be on the order of a few years, and $580 USD for a NIJ Class IV vest for every soldier every 3 or so years isn't that much money when you put into the perspective of Military readiness.