SMP on desktop is at least 22 years old. I have a dual Athlon MP (actually XPs with pencil lines) board and procs from 2001 sitting in my closet...the Antec TruePower 430W PSU for that system still works, but now relegated to a low-power i3-2100 print server system. SMP itself has been around since the 1960's.
I had SMP systems on 486, P54C Pentium, Pentium Pro and finally Pentium 2. I went back for the Athlon X2, Core2Q and a succession of i7s. The only generation I really skipped was the dual Thunderbird Athlon platform you had. I'm not going to pretend that anything from the Pentium 2 era or before was exotic, because it was. But the concept of threads and thread-aware software has been around on x86 since at least the late 80s.
Again, I don't know exactly what Adobe Lightroom specifically is doing in this case, but from the image previews, it looks like it's grabbing and operating on a maximum of four images at a time and I don't see any options in the software to tell it to use more than that. It doesn't look to me like it's hitting the GPU for very much at all of what it does, which suggests to me that it might not be tuned well for an abundance of hardware resources
Whatever you use CPU performance has moved on since 2019. Maybe you are awaiting the INtel Meteorite lakes (14th) or AMD Ryzen version 5?
Sure, but I do own a newer desktop platform, the R9 5900x, which is objectively no slouch in overall performance itself. For lightly-threaded workloads, it can be ~20% faster, but it's not more appreciatively productive
than the Threadripper because I can do both video and photo editing at the same time, and IMO neither AMD nor Intel's current-gen options are making me leap up and slap together anything newer.
I'd like to see a new enthusiast platform, but Intel has seemingly dropped that option and Zen 4 Threadrippers aren't really available cheap or used yet and IMO ALSO aren't so much faster that I'm willing to consider the investment. So yes, I'm waiting for something newer at this point.