For much of this decade, Intel was happy to have very limited upgradability for their sockets. Bought a Core i5 2500K? Want to upgrade to a Core i5 6600K? You're out of luck. You need a new CPU and a new socket. It wasn't always like this. But with AMD floundering with Bulldozer, Intel was left with minimal competition. They could get away with anything, and so they did. By the way, I love writing these more technical posts, though I know the audience is for these is limited here.
In 2016, AMD announced their brand new socket AM4 would be relevant all the way till 2020. This was a bold claim, and not many believed it. There wasn't much attention paid to it though, since neither did anyone believe AMD will be competitive in the near future.
But they were the year after, with Ryzen. Ryzen 2nd gen was even better, and a bonafide competitor to Intel. Ryzen 3rd gen blew Intel away in pretty much every workload, except for some niche ones such as high-refresh rate gaming.
Amazingly, AMD stuck by their promise. It's not perfect compatibility, mind you, with some understandable exceptions. But if you bought a X370 board to run AMD A-Series, chances are it'd also run Ryzen 1st gen in 2017, Ryzen 2nd gen in 2018, and Ryzen 3rd gen in 2019.
This was a remarkable achievement, and a paradigm shift, while Intel kept gouging their customers with new motherboards every generation or two. It also opened up new possibilities. For example, you could start off with a budget $50 CPU, or even lower on the second hand market, and then upgrade all the way up to the incredibly $750 Ryzen 9 3950X - a CPU that obsoleted Intel's entire lineup. Personally, I've gone from 1st gen Ryzen 7, to 2nd gen Ryzen 7 to 3rd gen Ryzen 9 on my Asus X370 Gaming motherboard. Everything works just fine, though it does lack some of the new features.
A week ago, AMD made a shocking announcement, saying only the X570 and B550 boards will support Ryzen 4th gen. Of course, outrage ensued. Some argued that AMD had fulfilled their promise of supporting products until 2020, while others pointed out Ryzen 4th gen was also due to release in 2020, albeit towards the very end of the year. Many complained this was all due to AMD now abusing their new found dominant position in the market, just like Intel did. That isn't fair, though.
There's a reason Intel dropped upgradability. Yes, they want you to buy a new motherboard every time, but moreso, it's just bloody hard to build BIOS that support so many CPUs. AMD learned that the hard way, and it turns out the regular 16MB BIOS in most motherboards simply cannot hold the data required for 5 generations of CPUs.
Fortunately, AMD has since backtracked, and has now vowed to support 4th gen Ryzen on older motherboards wherever possible. There's a genuine technical limitation here, as mentioned above, so it'll be up to motherboard makers to support 4th gen Ryzen using two solutions - replace support for older CPUs with 4th gen Ryzen, or if the motherboard supports 32 MB BIOS, then there'll be no issues there.
In the end, AMD has done the right thing, and has gone out of their way to support as many motherboards as possible for 4th gen Ryzen. They made a blunder by announcing they'd drop support completely, but at least they are doing what they can.
Still, AMD hasn't a 100% lived up to the promise they made in 2016. They simply did not foresee the challenges of such a long-term support for one socket, though the consumer doesn't care - they made the claim after all. All that said, they have done a solid job, far exceeding what Intel has been able to offer.
Going forward, I don't think AMD will be making such forward-leaning statements, so as to not shoot themselves in the foot again. AMD's next socket, let's say AM5, will probably release in 2021/22 with Ryzen 5th gen. When that happens, they are definitely not going to make statements like "AM5 supported up to 2025" like they did for AM4. Instead, it'll be a more vague commitment to support future platforms. I'm sure they'll try their best to actually support it for several generations, but AMD have learned the hard way that grand promises are sometimes just too much of a pain to respect.