AM5 is supported until 2026, possibly longer, but AMD have promised they'll stick on AM5 throughout 2025, and just like now - they're still releasing AM4 chips 18+ months into AM5's lifecycle.
LGA1700 is dead next-gen. It's already on it's last legs as Intel recycle AlderLake for the third iteration, which is one more 'generation' than Intel typically last for. I guess the reason it's 3 'generations' this time is because 14th Gen is a lie, it's just 13th gen silicon in every way with a new label on the box.
I think this thread mostly agrees that getting a CPU or GPU to remain relevant for 10 years is fruitless, but you can get a decent AM5 platform and go for a few years with your first CPU and GPU, and then when AM5 finally gets dropped (probably 2027 or 2028) you can get whatever the flagship chip is that will fit in your motherboard.
Same rule for graphics cards - buy something sensible for your games, settings, and monitor you have
right now without overspending, and replace the GPU when it starts to disappoint you. It might be 2 GPUs over 3 years, or it might be 3+ GPUs. The point is to
not overspend because new features will leave previous generations behind and you'll miss out on stuff.
- Get a good PSU, probably 1000W to give you some headroom. Corsair RM1000e seems like a reasonable option with good reviews.
- Get a Good 2TB primary SSD like a WD SN850X for your OS and games, then in a few years when your library of installed games gets too big, plug a 4TB SSD into the next M.2 slot and migrate your games library to that (many launchers support multiple libraries and Steam is great at moving games between them).
- Get sweet-spot RAM today, which for a 7800X3D is DDR5-6000-CL30 with an EXPO profile. Yes, the next CPU might be able to use faster RAM, but you're only going to be missing out on a couple of percent by using slower RAM. My experience with AM4 is that DDR4-3600 was the sweet spot but putting new CPUs in older systems with 6-year-old DDR4-3000 RAM is within 5% on average.
This is just my opinion but I think a decent midrange board with good case, PSU, SSD can carry you through at least 1 major CPU upgrade and several GPUs.