Once the vehicle starts nickel and dimeing you with repairs here and there frequently, its usually better to get into something else, or spend the time and money to frame-off rebuild what you have
I don't agree with this logic. Sure, it may be time to do some PM but if you know what to look for and can review and update the truck methodically it can be made comparably reliable with an effort far short of a frame-off rebuild. Maintaining an old car isn't rocket science, you just replace any worn out components and possibly any that you can anticipate failing in the foreseeable future depending on how far you want to take it. Also, no vehicle is ever further than one of many single components failing away from being disabled, even new ones.
In some ways I think that older cars can be made more reliable than newer ones largely because their flaws are more easily known via the shared experience of operators/owners and the fixes are typically well documented too. Newer and new cars may have big flaws in engineering/design that aren't widely known yet and that may not show up until year/mile x...
To some extent with the 80 series (or any car) total miles don't really matter and current condition is what you should assess independent of miles. I'd also say that miles you've driven are better than miles someone else drove, but whatever. Drivetrain components can be replaced or serviced. Frame fatigue should be minimal and likely failure points pretty easy to repair. Quality parts are still easily available in my opinion, esp. when you factor in used sources, and these are pretty easy to work on. 80s aren't the most efficient and they are a lot more complicated than say a lightweight 2wd rig but 80s are great for what they are and can certainly be reliable, even with 400k+ miles on them. If an 80 is what you want and can afford then keep it, maintain it and enjoy it.
There are people keeping less well made, less well supported, older, etc. vehicles on the road for far longer than 400k miles and driving them all over the place, it's not that big of a deal. There are situations where achieving reliability is more important like when you are in the backcountry or even foreign countries where support and safety net availability may vary but that's to be assessed by the user and what they are up to. I personally have driven old, air-cooled VW Westfalias all over the US, Mexico and Canada for decades and have never been stranded, etc. and those are on the other end of the spectrum from an 80 series LC in terms of build quality/reliability/etc. The main, perhaps only, thing that an 80 has working against it is it's complexity relative to 2wd rigs but it's still simple compared to most if not all newer 4wd rigs.