Apart from capacitors, I can't really say that motherboards just die from old age (not taking the ESD beast into consideration here).
Plenty of XT's and AT's are still happily churning in basements, attics and other computer enthusiasts' dwellings. I suppose that those poorly made who were thrown onto the market to make a quick win, have been smelted already.

Most components pre-94 (my guess) were made to last. The price of the equipment in the 80's and early 90's is proof of that. Mass production and mass use made personal computer into commodities to be discarded after a few years' use. That squeezed the manufacturer's profit and so the compensation went to mass production and the use of leaner production methods. The different scandals that came to light in the 90's and 2000's (caps, illegal price fixing between SDRAM producers, etc) are clear examples of an industry that went massively mainstream.

Still, quality products stood out, and they still do. Every time I hear the beep at POST and see the boot screen of one of my oldies, I bow (metaphorically speaking) for the craftmanship of the designers, engineers and factory workers. I suppose a car enthusiast has the same feeling when he hears the engine sputtering to life after cleaning the delco of his Beetle 😀

Testing will of course always be necessary, and that's part of the fun of this hobby. And most of us here can't spend the time we'd like on this, so we're with you on that one.
I'm not an expert on AT PSU, so I'll leave that to my colleagues. Those that I have in my 486's and P1's do still work without issues, but I do not know if I should be concerned or not.

PC Chips seemed to make the cheapest, worst socket 7 boards (at least it was their reputation), and 3/3 of mine still work fine (1 m535, 2 m590lmr)
but in the end it's kind of random it seems...

My rules are :
- keep it as cool as possible
- give it good, clean, ripple-free power

Doing so will make capacitors last longer and reduce chances of hardware failure .

If you must use vintage power supplies, make sure they are working in spec, otherwise repair/refurbish (or have them repaired) or replace with high quality, sufficiently beefy new ones .

The problem when getting old boards is that most of the time you don't know about their past history, under which conditions/environment they ran or how much "torture" they've been subjected to during their lifetime. What I generally do after checking it works, is clean it thoroughly and subject it to some stress testing. If it passes, I consider it good to go (though this is no guarantee that it won't fail in the future).

Regarding capacitors, this is mostly a problem with boards from the plague era (early to mid '00s) or some specific cases (those dodgy SMDs on Amigas or TurboGrafx for example)... but for earlier stuff such as 286/386/486/early P1 it's not a problem, given that those boards usually have only a few small electrolytics (or none at all). Sure, a tantalum may decide to explode once in a while, but usually that happens as soon as you get the board and power it up for the first time in god knows how many years. So a newer system is not necessarily a guarantee that it will last longer (ask yourself how many NES consoles from the 80's still work today compared to 2001-2002 P4/Athlon motherboards?).

As for power supplies, whether you use AT or ATX, always stick to well known brands (Delta, Astec, Lite-On, ...). They may fail at some point, but it's unlikely they will take other components to the grave with them. Also, in my opinion, crappy ATX units are more dangerous than AT ones because of the standby line, which is one of the more well known hardware killers if it goes wonky.

I've found that most boards that were maintained in the original case they came built in are the most reliable and least DoA. The key is finding something that hasn't been ESD damaged by improper handling (causes degradation in the long run). Then properly clean the board and remove any failing batteries. Proper storage is also key, keeping the board flat to prevent bending/warping over time, in an ESD safe bag with moisture prevention. Of course replacing electrolytics and other failing components also helps extend board life. Keep it running cool and don't overclock.

SPBHM wrote on 2020-05-01, 22:00:

PC Chips seemed to make the cheapest, worst socket 7 boards (at least it was their reputation), and 3/3 of mine still work fine (1 m535, 2 m590lmr)
but in the end it's kind of random it seems...

My M919 outlived my MVP3-based SS7's...

I can't say I've had great experiences with PCChips in my case. One of m912 never worked even though it looked perfect, same goes for one of my m919 and my m915i broke. I also have a m537 that doesn't seem to be stable.

I have 3 great working Pcchips M912 AKA Amptron DX6900 boards. All three have real cache, one v1.4 and two v1.7. Now there is also two other Pcchips I own that are horrible in one way or another ! I think some of their boards were very well made and others were built to be as cheap as possible, just depends on the model and version #

Horun wrote on 2020-05-02, 01:54:

I have 3 great working Pcchips M912 AKA Amptron DX6900 boards. All three have real cache, one v1.4 and two v1.7. Now there is also two other Pcchips I own that are horrible in one way or another ! I think some of their boards were very well made and others were built to be as cheap as possible, just depends on the model and version #

My old PC Chips M550 ended up being used reliably by a friend's sister for several years into the early 2000s . I think they only got rid of it because it was too slow by that point . When I had it, it was rock stable and overclocked with a Pentium 150MHz at 187MHz (2.5*75MHz) without any issues .

440bx wrote on 2020-05-01, 20:39:

Is there any way to get long-term stable, reliable hardware for slot 1 / socket 7 or even older builds? Not nearly-death board for booting and shutting it down...
The best way I discovered for retro digging is Cedar Mill/Conroe board with AGP slot to run W98 with all available drivers.

Short answer: no. Unlike with other stuff, you can't see wear on electronics, nor how it was handled in the past (ESD, heat, etc.). It's always a gamble.

More complex answer: even brand new, old-stock hardware is not any more (or less) reliable than used one. It seems to be a common belief that untouched hardware won't have aged - apart from caps that may leak, of course. But any semi-conductor does age, even when not used. A new, never-used chip may just no longer work after some decades in storage. Any sort of ROM will also forget its content sooner or later, with masked ROMs having the greatest lifetime and flash-based ones the shortest. That's all because of physics, which I'm not going into.

Bottom line: just use whatever works and enjoy it. Accept that it does not last forever.