Forgive me kerri; alas! But to answer this question, I must beg that it belies an assumption - ie, that the most important thing in warfare is the quality of your equipment.

Surely it is so that equipment helps the soldier to fight, and some of those things (the jump boot) are marvelously well designed now. But Armies, Navys, and Air Forces standarise the equipment around logistics pipelines for what can be incredibly complex machines, along with all the intricate and interlocking training systems needed for people to build, repair, and use those machines. Cost. Availability. Most importantly though, doctrine will affect what equipment is considered good to go and what needs to be replaced lickity-split. Does that make sense? Everything goes in the end, but the stuff that works with current doctrine tends to stay put until it's needful.

Hence the jump-boot won't have changed, but you bet your ass the armour-systems on a HUMVEE will have when the Americans switched styles from heavy-vehicle warfighting to light-vehicle patrolling in the Middle East.

Take the Brown Bess musket, talked about further down. It saw over 100 years of active duty, because it was perfect to equip a large formation of close-moving soldiers battering away as fast as they can at other large formations of close-moving soldiers. Rifles - which had been invented but were limited to skirmishers' weapons - did not support the doctrine of the Force which employed them, and were expensive and difficult to use. The rifle gained many more supporters when those factors were reduced... and doctrine was altered to sui thet new circumstances. But it was only once the doctrine changed - essentially only after certain groups had tried it out on a limited scale - that the rifle saw widespread adoption by militaries. Does that make sense?

On the other hand, a great example of quality kit not working with doctrine and being randomly replaced is the A-10 Warthog. That is a fantastic plane and clearly perfect for its role; CAS. The problem is that the Air Force most decidedly does NOT wish to be strategically subordinate to a ground-role. They wish to be relevant all on their lonesome - flying fighters to gain air supremacy, and then bombing the hell out of things with conventional or Atomic weapons to force a result. Who needs Navy/Army? That strategic worldview is also the only thing justifying their existence as a separate service outside a Corps of the Army. The US Air Force has therefire tried to get rid of the Warthog several times because it doesn't mesh with their conception of what the Air Force is and what it should be doing - in fact it is downright dangerous to that vision. Does that make sense?

I am sure I am teaching my grandmother to suck eggs, but I sometimes wistfully think the focus on technology obscures the way that war is fundamentally about human beings. It's worth considering, anyway.

Sort of. What you mostly see, at least with 18th through early 19th century small arms, are minor changes made every decade or two to a design that's much older.

The Land Pattern Musket, famously known as the Brown Bess, was in use in various iterations from 1722 to 1838. The action remained the same; the loading method; the firing mechanism; the caliber; but minor changes were periodically made in length, weight, shape of stock, etc. As these firearms were not mass produced in the way 20th century firearms were, and they lacked interchangeable parts, it was really no problem to make minor tweaks whenever a new model was produced.

The French Charleville musket has an even more tortuous history. The original model dates from 1717, the last French model to 1777; it would be in service well after the Napoleonic Wars. But the development diverged well before then. The Americans were given significant numbers of Model 1766 Charlevilles during the American Revolution, and a domestic model based on it was later produced, the Model 1795 Springfield, which would form the basis for American musketry for the next fifty years. Subsequent models featured minor improvements: the Model 1816, Model 1835, and finally the Model 1842, which featured a percussion ignition system.

I'm sure there are countless other examples from different periods that will be brought up, but Ill speak form my experience as a Royal Navy historian. Naval ships in the 17th, 18th and 19th century could and did see decades of use. Take HMS Victory as a famous example. She was commissioned during the Seven years war and launched just a few years after its conclusion in 1765. She served through the American War of Independence and then of course famously in the Napoleonic wars as Nelson's flagship at Trafalgar in 1805. She was only withdrawn from active service in 1812, 47 years after she was first launched (and she is still in commission today as a flagship, the oldest commissioned warship in the world, but that's a story for another post). The reason ships like Victory could serve as long as they did is that the basic methods of Naval Warfare did not undergo any significant change (yes there were of course small changes) from the 17th to the mid 19th century. During the late 19th and early 20th century, as a counter example, new designs and new technologies, such as that of HMS Dreadnought, could render ships obsolete within a couple of years of construction. I don't want to speculate too far on the examples that you mention, as they are outside my range of expertise. However I would guess that the same basic principles apply. As long as there is a military need for the particular type of weapon in question, and the weapon currently in use fulfills the requirements of situation, there is little need for radical design changes

Sources: Precise dates for Victory are from here: http://www.hms-victory.com/history

I see you're flared with Caribbean Piracy, so you might know these books already, but some excellent sources on 17th, 18th and 19th century Royal Navy are:

Black, Jeremy. The British Seaborne Empire. New Haven: Yale University Press, 2004

Marcus, Geoffrey J. Heart of Oak; A Survey of British Sea Power in the Georgian Era. London; Oxford University Press, 1975

O’ Hara, Glen. Britain and the Sea; Since 1600. New York: Pelgrave MacMillan, 2010

Rodger, N. A. M. The Wooden World: An Anatomy of the Georgian Navy. London: Collins, 1986

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