Intel actually intended for LGA1151 to remain unchanged for Coffee Lake but found out late in the testing process that many existing motherboards did not have enough power delivery capability to support the planned 6 and 8 core parts. Hence the decision to lock them out in software only. They are probably aware of the bad optics but decided that it’s better than trying to deal with the RMAs later.
It’s very similar to what had happened in 2006 when the 65nm Core 2 series were released in the same LGA775 package used by 90nm Pentium 4s, however the former mandated a specific VRM standard that not all comtemporary motherboards supported. Later 45nm parts pretty much required a new motherboard despite having the same socket again due to power supply issues.
AMD went the other route when they first introduced their 12 and 16 core parts to the AM4 socket. A lot of older motherboards were clearly struggling to cope with the power draw but AMD got to keep their implicit promise of all-round compatibility. Later on AMD tried to silently drop support for older motherboards when the Ryzen 5000 series were introduced but had to back down after some backlash. Unlike the blue brand they could not afford to offend the fanboys.
P.S. Despite the usual complaints, most previous Intel socket changes actually had valid technical reasons for them:
- LGA1155: Major change to integrated GPU, also fixed the weird pin assignment of LGA1156 which made board layout a major pain.
- LGA1150: Introduction of on-die voltage regulation (FIVR)
- LGA1151: Initial support for DDR4 and separate clock domains
This leaves the LGA1200 as the only example where there really isn’t any justification for its existence.