1) Socket 754 was indeed not very good, but at least you could put both 130nm and 90nm CPUs in it. its purpose was to make boards cheaper, so while its production might have been a long-term mistake, it was not just soem ill will.
2) Socket 939 was abandoned due to switch to DDR2, so there is no grounds for criticism at all. And moreover, it supported 130nm, 90nm chips AND dualcores!
3) Which brings up to the LGA 775. As was already said, there were numerous compatibility breaks. When Core 2 came, and then when 45nm Core 2 came (Penryn) - those both cut compatibility with older boards like Coffee Lake now, or like Broadwell desktop chips. But that was not everything.
775 boards made before introduction of dualcore Pentium Ds could not be upgraded with them and you had to buy new boards too. ANd I think there was one similar schenanigan when Intel raised current requirements - again, newer chips unsupported in previous motherboards. There might have been something with 1333MHz FSB but I am not sure.
I hope you see why I felt the need to defend socket 939. Sure, it was unpleasant for me that the dualcores for it never got cheap while the AM2 ones did. But that is a different problem - the platform's life was not cut short artificially, it ended due to migration to DDR2, which was already cheaper than DDR1 in 2006 and thus needed to be adopted.
You are right about 775, it had backward support, but forward support was indeed a mess, yet in those years AMD was seeing as the one who changed sockets too much, as 775 remained static and supported from DDR1 to DDR3. It was a technical reason i know.
I remember i started with 754 and them switched to AM2(i had a Xp1700+ before this), while i agree with you that the 939 to AM2 switch was because of DDR2, people was not happy at all back them. 939 was short lived, and as you said, dual cores were expensive. 939 should have never existed at all,
Personally im not happy at all with AM2 either, neither of my two AM2 boards recibed bios update for AM2+ cpus (Asus m2n4 sli and MSI K9AGM2), and AM2 cpus had bad TIM problems, TIM dried after 1+ year use on my X2 4200+, and many, many people had this issue as well.
Then i switched to AM3 (Gigabyte GA-MA770T-UD3) and Phenom 2 X3 710, nice cpu, but i got screwed again with AM3+, switched to Intel the exact same day FX8150 came out(brought a 2500K) and lasted for 6+ years of constant abuse, OC, and near top of the line performance.
So you can see why i dont trust AMD too much and i find funny to read that AMD is better at keeping motherboard compatibility. And thats not considering what they did with FM1 and FM2, that was 754 and AM2 all over again.
I decided to give AMD a second chance with Ryzen, i have a R7 1700 now, but i really dont trust anything they said, im fully expecting them to release AM4+ with back support for wharever reason and forward support depending on oem.