At the very low end you have BSO or Bicycle Shaped Object. Those should be avoided. At the very low end you get parts that are not even serviceable.
At a minimum you want name brand (e.g. SRAM, Shimano) components. They are serviceable and you are going to be able to find replacement parts. A bike with low end named brand components will start in that $400 (US) range.
A low end bike is going to have low cost drive train components as well as low cost tires, wheels, post, bars, ....
Yes you are going to have stuff wear out faster on low end bike.
If you are having problem with a seat post that is like a design flaw. You should expect a working seat post on a low end bike.
If you want higher end components it is always cheaper to buy a bike with those components up front. Replacing components is expensive and labor intensive.
You almost never get a bike you never change any component. Even higher end stuff wears out. Look for a good frame and a drive train at the target level and target budget. Beyond mid range you are not going to get more reliability - the expensive stuff is for weight and racing. Like in Shimano in the road line 105 is a very reliable.
At even the $800 point you are getting a mid range drive train. Well more than the lowest level drive train components.
At $800 you are not going have $400 wheels and a $100 seat post.
Even at $1200 you are not going to have $400 wheels and $100 seat post.
Seat is lower end on almost any bike as on a higher end bike it is almost always changed out. Higher end bike often don't even come with pedals. Even on a mid range bike you are not going to find an expensive seat post as many people like an offset - it is a component that is often changed out.
Wheels are typically going to get better as you spend more. If you put a lot of miles on a bike you are going to wear wheels out. When you upgrade then put a nice set of wheels on the bike. What you want is core bike that you are willing to put more money into consumables.
On a mountain bike never go full suspension on a low end bike. That is going to be a low end suspension you need to put money into and money that did not go into other components. Even the front suspension on decent mountain bike is going to be low end.
At $800 you are getting a decent bike. For light needs even $600 is a decent bike. At $1200 are you getting the stuff you really want - yes I like the wheels, post, pedals .... Beyond $1200 you are getting more into performance rather then reliability. That said a $2000 bike is probably more reliable / durable than a $1000 bike but not any where close to twice as reliable / durable.
Your best buy in my mind is used $1200 bike you got for $800. Or a $2000 bike you got for $1200. The higher end bike always have higher discount. You get people with money that buy a new bike every few years.