This post is prompted by @Cathul Thuku's post on economics, but is actually not going to touch on that much. It IS going to focus on the One Sith-Republic war and what that region of the galaxy is looking at when it comes to armament following 5-7 years of constant warfare. Obviously you can't simply ignore the Galactic Alliance, so they get factored in. There will be a great deal of supposition and assumptions present, and if you've got an argument in the contrary please bring it up.
Several blog posts ago I postulated the total force of the One Sith Navy to be around 22,000 warships. I think this number still works, but with the corollary that much of that number is made up planetary defense forces comprised of ships that have served in that role for decades.
These 'legacy' ships (the pirate frigates so often the foe in dominions, or the ISD's that seem to lurk around every corner, and so on and so forth) are not simply a fleet in being, they fill a valuable role in any populated system. From defense against pirates, to deterring smuggling, keeping trade lanes open, ensuring lines of communication, and so on. In times of war they are merged with the fleets of whatever major faction owns said system for the defense.
Obviously not every system is going to have ships like this, but i think it's safe to say that most of the core/colonies/mid rim systems would. Most of those fleets would be now have been destroyed too.
Quick raid to hit a shipyard? It's the defense fleet that dies to buy time. Invading a planet? Gotta get through the defense fleet. Want to hurt the economy? Cripple the defense fleet, the cripple the merchants bringing materials to repair them. So on and so forth. Hell you could keep reasoning this out to postulate that the whole naval structure and concept of operations changed in order to sustain total war on the massive scale it must have been between the OS and GR but I'll leave that for another day.
Given all the warfighting going on 'off screen' I think we're looking at around a 15-20% attrition rate per year. Obviously that would be unsustainable but you can easily factor that most of that is replaced or repaired. Or at least it would have been for the OS. The GR, not quite so lucky. At the time i wrote the last post (about a year ago) the GR still had a lot of territory. They since been cloud-broken several times, and I think it's fair to say that they can't keep up with attrition.
Fast forward five years. The One Sith have had a steady 3% attrition rate for the first three years, then peaking at 6% as they redouble efforts to conquer the GR. The GR have had an increasing attrition rate as follows.
Year 1 - 5%
Year 2 - 8%
Year 3 - 12%
Year 4 - 17%
Year 5 - 23%
It's sort of excessive but whatever, that makes for an interesting read. It leaves the GR with approximately 10,800 ships and the OS with around 18,000. Honestly those numbers might well be unfair to the OS, but we'll let it be because the GR had exceptional commanders at the time and would be defending an increasingly smaller and more cohesive bit of territory.
The way the math works out on this by the way is that both sides have lost at least 10,000+ ships in this war. They're just able to replace most of those. When you want to start talking about the cost, it's worth considering that even a damaged ship might need a huge amount of resources and time devoted to it for repairs. When you figure that you're looking at new constructions, you start to get a sense of just how HUGE this economy must be even for the 'small' 22,000 ships we're talking about.
This also doesn't factor in invasions and large battles, which would cost a higher amount of ships lost and be harder to recover from. Of course most of the fleeting done on the board is 10,000m of ships, usually 8-15 vessels, or six thousandths of a percent of total.
For the OS, you're looking at controlling MORE territory and MORE planets with a lot fewer ships. That helps make sense of how the Galactic Alliance can turn up and punch your nose in twice. Sure, you've got the Omega Pyre fleets (say, 6,000-7,000 ships) but for a faction with little centralized authority you're probably not looking at more than another 10,000, and those mostly smaller vessels.
So the Galactic Alliance does a series of feints along the border to harass OS forces and cause them to disperse their fleets. When they do strike, at Dulvoyinn, it's a massive engagement but they handily win and one can assume are able to cut off a lot of smaller units and destroy them piecemeal.
At some point he Sith are just going to need to have ships in more places than they have ships for. At that point the GA fleet (bolstered no doubt by ships fleeing the GR) will have an ever-increasing numerical advantage and lo, there's roughly the math on they the OS are collapsing.
But holy hell look at the cost. Over the course of five years you have what, 20,000 ships destroyed? Literally millions dead from crew alone, the resources of what must be ENTIRE WORLDS strip mined in order to fund the production of ever more star destroyers. Is it all a drop in the bucket or are we looking at economic upheaval and social collapse in the core?
I don't know, that much is up to the writers to decide. Maybe I'll write a blog post about it later.