I've inadvertently stocked piled a bunch of DDR4 memory over the years. Usually bought them on an impulse when I saw a good deal, hoping to use it later when someone in the family/friend wanted a new gaming PC or for my own homelab projects. I'm happy with just using DDR4 at this point. The motherboards/CPUs are still relatively cheap either new or on secondary markets. With a decent GPU, the performance difference between DDR4 vs DDR5 is marginal. Most people won't even notice it. My point is, if cost is a factor for a new build, then stick with older hardware. Especially if your use case is gaming or home server.