Military expansion is the most rewarding expansion and building more ships furthers this goal.
Not if every NPR you meet typically are 2-3 tech levels ahead of you it is not... 
Try conventional starts with 10-15% tech progression and 5% survey speeds and you will know.
I think your ideas are a bit simplistic... in Aurora military expansion is not always that easy or even wanted. The amount of resources to invade a decently defended colony can be extremely costly, resources you can spend more wisely elsewhere instead.
It is all about juggling different priorities all the time, there is no black and white... mostly grey.
It also depend on what type of campaign you play.
I think I can agree a lot with the above and with this:
It will be very difficult to expand the economy as fast as you can expand your military yards. I believe you are not constantly expanding your military yards 24/7 outside retooling?!?
I also play with about 10-15% tech progression (conventional start) and that will usually have a pretty strong implication on this... it is way easier to expand the military capability by expanding few shipyards to the point you can't build ships 24/7.
It is a very interesting conversation and I agree with many part of it. You need to have a military because it's just simple as that. Also you cannot have your military to eat up all your budget and resources. Only things I don't agree is this and I break them down:
From a strategic point of view it can be quite effective to have a strong production capability that you don't use as military ships will cost maintenance over time and that also cost you resources. If you are not at war you don't need a massive offensive fleet.
I personally think, also considering the new claim mechanics that your fleet should always have an offensive capability ready for action and also to scare visitors away. Also, please don't forget that even if you can build a ship in no time other thing is to train her crew at an acceptable level to make that fleet actually combat ready.
As long as you can defend your territory you don't need a massive offensive fleet on stand by all the time, that massive fleet is a huge drain... I mean I can go many decades without war quite often. Some wars are just stalemates or low intensity any way so don't need a huge fleet. Quite often going on the offensive can be way too expensive as you already are exploring and expanding in other places and all effort already goes 100% into that. So even if you did win after a huge expansion of fleets and ground troops you will not be able to take advantage of it as your expansion already is at 100% capacity elsewhere.
Half true half not. It may be entirely possible that you are having stalemates or low intensity wars because you are just not ready and probably you don't really want a war in first place. Beware I always prefer an ally than an enemy, but I prepare for both. Obviously going huge it's insane and on that I agree with you a lot. Side note: I always, I mean always build ground units. Number 1: they are cheap but still take time to train. Number 2: you lose an insane amount during invasions and you need even more to "pacify" regions. They always come in handy.
I agree with you on other point here below though. I personally don't start a big military program until I've found proof of alien existence whether it's ruins, wrecks or lives form. The best game I had was when the first proof I had was a non TN race. If I close my eyes I can still hear them screaming when our ships cast shadows on their world.
I have had games (almost always conventional starts) where it takes 50-60 years just to get out of sol, let alone meet a hostile alien... why would I need to build 100kt ships per year after 20 years, that would be a huge waste of resources and production at that time. I probably could do that it I wanted to, but I rarely want to.
I think we agree in even the things you pointed out. I never meant you should not have ANY offensive ships standing around at any time, especially if you know there are hostile aliens out there that can potentially attack you.
What I meant was that you don't need a huge fleet (and ground forces) that eat up your resources for invading an enemy capital or their colonies standing around all the time when the resources to build them are massive let alone maintain it over time. Especially if your enemies are stronger than you are. It is a whole other matter to keep a manoeuvre force strong enough to attack and raid enemy space in preemptive attacks or to bolster certain part of your space if any hostilities break out.
You have to judge what is a good balance between maintaining fleets and ground forces and how much of your yearly wealth budget goes into ground forces, ship construction or colonial industry and mining efforts and technology advancement. You also need to balance mined resources versus consumed resources in addition to this.
I try to strike a good balance and the more I put into construction of new colonial efforts the faster the economy will grow and the more general military power I can maintain in the end which include both general defences and manoeuvre elements.
It is not entirely uncommon for me to scrap some ships after a bigger confrontation as I simply have more ships than I need for general defence. I might start with older ships but sometimes even newer ones can get scraped too, it depend on the ship type. If you have the components left you can quickly build new ships if you need them for not a huge difference in cost. You have to judge the situation you are in if this is something you want to do or not.
You definitely have to consider the training level of the crew as well. I mostly use level 5 training quality of my military crew so they will start out with a fair amount of crew training to begin with, but still you have to consider this as a whole as part of the decision making.
Ground forces are quite expensive to build as well, especially if you want to constantly "upgrade" them as you develop new technology. So here I do the same as with ships. Build up good defences and some good manoeuvre forces for rapid deployment when needed. I don't sit with huge invasion forces all the time... I think I can use my wealth for better things in the mean time.
