the aga chipset is quite robust even being old, not much fails in 4000s other than caps,ram etc-battery damage is also a bad one. All the ttl level chips can still be bought from places like mouser etc. having a motherboard and 3640 capped runs under $100 so that doesnt seem crazy to me. It sounds like you gave up due to lack of experience. Sadly all the info to make these machines go is on the net. I am confident i have enough parts and such for my 4000's to easily outlive me.
Well let's see, I didn't see anything that looked like battery leakage but I imagine it can be hard to detect if well cleaned. All batteries had been removed and nothing looked to be discolored like pictures I've seen of leakage. I built my own power supply with a modern power supply with correct A4000D power connector to test to make sure it wasn't a power supply issue. I replaced the RAM, I had a box of the correct SIMMs from other systems. Did a general cleaning and made certain all the socketed chips were seated properly, etc. So I guess, sure I stopped from lack of experience on what other components to swap that were readily available. I don't know where you are getting the motherboard and 3640 re-capped for $100 but all the places that I checked with wanted a lot more than that. I had sent an accelerator card to France once and spent a big chunk of change to get back a board that didn't work. I suspect that a recap is what was required.
The one other common failure on the 4lk is on the 3640 accelerator, when a cap leaks it often eats a trace under the cap. its not too hard to fix usually. Of course there are the variables of what stupid people do to these machines to damage them. I have seen some carnage caused by people who shouldn't be messing in hardware but didnt have the sense to realize it.
I always stay within the frame of what I know how to do and not to do. I wouldn't try to replace capacitors or repair a trace without additional training. My soldering is limited to tinning leads for joining wires and joining copper pipe fittings. Wouldn't dream of damaging an old system by going beyond that.
I also think that there are a lot of people out there that enjoy the retro Amiga scene or at least watch to see where it's at that would not at all be interested in doing board level repairs. I still think that they are a valuable addition to the community but this might just point to the need for a source of retro AGA hardware. I hope Jens can come through and make a modern AGA motherboard with his stock of AGA chips.
You will find these old Hmos/Nmos chips will likely go on 60+ years.. the c64 is already over 32+ years old
The C64 also just got a brand new motherboard made for it so that you can populate the new board with chips. This is a great refresh and is something that I think Amiga desperately needs. I've got my fingers crossed that Jens can pull off some form of AGA motherboard with modern video output, I/O(USB), and SATA support. That would be amazing and certainly extend the lifespan of retro Amiga hardware for those of us who are good technicians but not hobbyist electrical engineers. ;^)
Keep on Amiga'ing.