The only rifle that never breaks down or needs to have parts replaced is a rifle that doesn't get used, no matter what brand you start with.
What you get with a top tier AR is likely to be a rifle built with parts that will last longer and perform better. If you're looking for a rifle to buy, ask yourself the question of what you're going to be doing with that rifle and then make an assessment of what is going to be good enough for you.
Some don't want to spend more money than they need to. Some are willing to spend more money for either the pride of ownership or the peace of mind that comes along with buying something better than "good enough." Then there are those that are willing to spend the money to buy top tier stuff because it absolutely, positively has to work.
So, the question still remains. What is good enough? I can't answer that for anyone other than myself. As has been mentioned, there is no quantifiable data that clearly states that "lesser" rifles break down or fail more than "better" rifles.
I know that I think what I think, after having had to work on more ARs that I can remember for various reasons, because of my personal experiences.
Sure there are budget priced rifles that will, for some undetermined length of time, operate the way they're supposed to; but when the time comes to start replacing parts, what you are going to replace those parts with?
You can go through the threads on this site alone. If you need to buy a replacement bolt, what are you going to buy? Are you going to buy one that is used in a budget rifle or are you going to try to find something that is machined of milspec carpenter steel that's been shot peened for strength and magnetic particle inspected with a strong tool steel extractor?
If you need to replace the barrel, are you going to replace it with a budget barrel of 4140 steel or are you going to replace it with the best barrel that you can get? Maybe something of a higher grade steel that is hammer forged, high pressure tested and MPI with a good chrome lining?
Maybe some guys are looking at the way of choosing which rifle to buy back a$$wards. Maybe a good way to go about it is to figure out what ingredients you want and buy a rifle made of those ingredients.
When I was really into ARs there were particular parts that I looked for because they will last longer. In some cases, a lot longer.
For a non precision rifle, I want milspec steel and prefer chrome lined. I want a barrel that is coated in the gas block area. Hammer forged barrels are nice. I really like the barrels from Bravo Company and Daniel Defense, although these brands aren't the only good barrels out there. I wouldn't get overly excited about things like 5r rifling and/or any special coatings. Neither are new. In fact, both have been around for a while and neither will turn a turd into a diamond.
A barrel is all about the steel and the manufacturing process. I'm not talking about being able to shoot sub MOA. Really, I've seen most barrels be able to accomplish this, given the right ammo. The real test of the quality of a barrel is how it behaves under stress. If you really want to see what a barrel can do, put 50 rounds through it quickly and get it good and hot and then shoot groups with it. This is when you'll see the difference between a good barrel and a lesser barrel.
I want a full auto bolt carrier group that's been chromed in the right places and made of milspec or better steel. The BCG is the hardest working part of the rifle.
I also want a milspec or better trigger. Milspec triggers have been hardened to a specified depth and, although they may not produce the best feel in the world, they will be more reliable.
Lastly, I really don't know if there is even a milspec on the small parts, but I buy things like springs, rings and pins from vendors that build milspec rifles. They are made from the proper materials and are the proper size. I get my stuff from Bravo Company and I say that they are made of the proper materials and are of the proper size because I've never had any problems with them.
If you don't think this last thing is important, you can do a search here on THR and see the number of threads of guys asking why their AR won't eject, or the trigger won't reset, or the bolt catch doesn't work the way it's supposed to, or the trigger pins are walking, etc.
For those guys arguing that the top tier rifles (and what makes those rifles top tier) aren't worth the cost or that top tier status isn't worth it because something made of budget parts is just as good, I have to ask...do any of you guys have any hard data?
Do any of you guys have something like a Smith and Wesson Sport, Stag or Rock River with 5000 rounds or 10000 rounds through it? Without having been cleaned? Have any of you ever shot an AR to a point where you NEEDED to change the bolt? Cam pin? Or even the rings? I'm asking because if you haven't, you haven't shot the rifle enough to be able to tell anything from personal experience.
I'm not trying to get harsh, but I'm hearing a lot of claims like "my rifle has never given me any trouble" or "it's a tack driver" or "I run it hard." Those statements are also pretty vague.