Greetings Charlie,
Lets take this in chunks.
Why are electronic products so hard to repair/why is it cheaper to just replace?
its not the fact that they are difficult to repair in all cases its finding the problem that is difficult, the components are getting so small, and the devices so complex that in something like a laptop or something there could be 20,000 components, thats a lot of time troubleshooting to find the one or two bad parts.
Narrowing it down to a specific board can make things faster BUT buying said board will probably cost you almost as much as the unit itself.
Because these things are mass produced the companies buy in bulk, and build in bulk, so price per unit can stay down, but buying one board means they have to ship it to you, and make sure it gets there, and its one piece that is taking a lot of their time to send to you.
Take, for example, the video processor board for an LCD television, the parts may only cost a total of 40-50 bucks, but if you want to order the board it will cost you almost the whole, if not more than the whole price of the television in question because you are paying for the people who designed the board, made the board, populated the board etc.
What is making product life cycles so much shorter?
The product life is shorter because of competition, everyone wants to sell as many units as they can, so they want the price to be as low as it can go while having the best features. To do this they use cheaper parts, if they were given the option of 2 chips one warrantied for 2 years at $0.50 apiece in bulk with a week lead time (time to get them from the seller) vs one that is warrantied for 5 years at $0.75 apiece but with a 6 week lead time they will go for the $0.50 one, because they can get it in a pinch, its cheaper, and their product is only warrantied for a year anyways...
There are also a lot more things to go wrong with the newer stuff, look at an LCD TV, the TV should theoretically last somewhere between 50,000-500,000 hours of on time (depending on manufacturer) but if one thing goes wrong before that it can mess the whole thing up, discounting dead pixels because those are inherent problems of all LCDs.
If there is a power surge on the lines then the lamps can blow, a whole slew of capacitors can get fried leading to video controller board issues, sound issues, input issues, etc. If one of the chips had a small defect in it when it was made it may burn out earlier than it was supposed to.