Where do Intex pools fail?

I looked into this upgrade and what I ended up doing was to keep my intex 2100gph pump, invest in a bigger 18" Hayward sand filter and increase the flow by adding a Hayward through-the-wall skimmer and return. Mine is a 15' x 48" Prism, and the original fittings were 1.25". I cut a new hole for the skimmer and enlarged the return to fit the Hayward return. I preferred that return over the Intex 1.5" one because the Intex one actually narrows inside down to something like a 1" hole, defeating the purpose. It also required a bigger hole in the pool wall. I decided against upgrading the pump because a decent pump will require a 220V outlet, which I have not yet installed -- and I'm not certain I feel good about installing close to a framed pool. A whole thread on this from last year... Upgrading the filter was easy and not super expensive, and the effects are striking. It ran the entire season last year, with daily swimming by 2 - 4 people, and we never had to do anything besides regular maintenance. No shock, no SLAM, no algae. Disadvantage is that now you no longer have an Intex pool, so you will have to get really good at adaptors.
To answer your other question, I found that the legs on the side with the pump have started to rust after one season. So the next season, I sprayed the edges of all lefs and frames with Rustoleum. This year, there were still some traces of rust, and I dipped the ends in vaseline before assembling. We'll see next year. The damage was never extensive, and we have some fairly regular and copious rains, but the part that rusted was only the side of the equipment, both sides. Interestingly, you can purchase replacement legs.
Mine is a newer model with the plastic connectors rather than the pin system they used to have. That seems to leave less components exposed to the elements.

There's not a lot to go wrong on the pool itself. Either the steel parts rust out or the liner wears out. Having proper pool chemistry helps with the liner. I've heard on newer models they powder coat the inside and outside of the supports. Overall, I'd guess a 5-10 year lifespan depending on climate.

I picked up a used sand filter and pump on facebook marketplace for $100. I then added a Haywood skimmer that I had to cut a hole for. This year, the liner was shot so we got a new Best Way pool. Still use the same filter and got a Intex over the side skimmer instead of cutting a hole.

The used filter and pump has probably paid for itself in ease of cleaning many times over.

I have an Intex Ultra Frame Pool 18' x 52" round that was bought and installed in 2014. we replaced all the framing this year using an Intex Ultra XTR Frame pool parts that was bought directly on Intex's Website. I replaced the pump 3 years ago with a Heyward 1 Hp pump, I still use the Sand Filter that came with the pool and I had to replace the salt water clorinator system 3 years ago as well. I installed a wide mouth skimmer in 2015 and PVC'd all the water lines. I have a great flow, vacuum works so much better and my water is always crystal clear. I don't see us having to replace this pool for many years ...

Alot will depend on your maintenance. As mentioned, chemicals have a lot to do with the liner. But also, the way you maintain the exterior. I hose mine down once a week as well as wiping down the legs. During the winter I wrap a good portion of the pool to try and protect it the best I can. So far 3 years in no issues.

I have a Coleman Vista 2 going on its third season. I noticed rust on most of the tops of the legs where they got into the T post connector. We're heavily debating on letting water out, disassembling one piece at a time, sanding off the rust and respraying it to make them last longer.

So far I don't see anything other than that being an issue. I did go for an overkill sand filter and cut-in skimmer. The skimmer makes a big difference.