I always see people make a big deal about needing new motherboard for upgrading CPU. I wonder how many people actually do this. I have built every computer I ever owned over the past ~20 years now. I have upgraded ram,GPU,HDD/SSD but in all that time never needed to upgrade a CPU while not also wanting stuff that my current motherboard did not have. Like newer version of ram, or new PCI-e version, or something else that simply a new CPU would not provide.
My experience is the features of my motherboard become outdated faster then my need for a new CPU therefore it just never seem important to me that AMD or Intel kept backward compatibility on there motherboards.
See the 4,1 Mac Pro, which shipped with 45nm quad core Xeons. You could flash that model to 5,1 and then upgrade to 32nm 6 core Xeons (which also supported 1333 DDR3). It was quite an investment, but one could move from 2 x 4C/8T to 2 x 6C/12T without shelling out for the 5,1. An odd example, but back in its day, it was a significant upgrade.
Back when I was right out of college and had limited funds, I used to only upgrade one part at a time. I recall dropping my Dothan 800mhz into a new motherboard that supported the Athlon XP and DDR. When the Athlon XPs got cheaper, I bought one and saw a noticeable performance gain. Granted, today, there isn't as much incentive to upgrade one part at a time other than the GPU, since typically the higher end CPUs mostly offer more cores to gain performance, but those gains are only realized in specific scenarios. In the single (and even dual) core days, the difference was very noticeable across the board.