To address each of your points in turn:
8GB is RAM was the minimum I was buying 4 years ago. Back then, it was because it was the sweet spot in price per GB. Unfortunately, in some machines it was the most that the board could support and so is now the thing making me ponder replacing the motherboards. Specifically, on my NAS box, because increasing the disks will increase the size of the deduplication tables, meaning that I'll need to increase the size of the RAM to get tolerable performance, meaning I'll need to replace the motherboard and CPU to be able to accommodate more RAM, meaning that I'll end up just keeping the case and optical drive - everything else is upgraded.
Swap out the hard disk for an SSD? The only machine I've bought in the past 6 years that wasn't SSD-only has been my NAS. The laptop I've just replaced had a 256GB SSD and it was replaced with one that has a 1TB SSD. Buying hard disks hasn't made sense for years unless you need a lot of storage that you rarely access (i.e. NAS / SAN uses), and even then adding an SSD for L2ARC makes sense (as long as you have enough RAM).
Upgrade the video card? I've not done anything that taxes the GPU in my old laptop, but then I'm not a gamer.
Not wanting to upgrade the CPU? You claim two bottlenecks. The first is disk to RAM. My laptop's SSD can do over 300MB/s sustained transfer and over 60MB/s on small random files. With a reasonable amount of RAM, the only limiting factor is the SSD write speed, because all of the working set lives in RAM. If you think that RAM to cache bandwidth is a bottleneck, then you're running some very unusual workloads. If you're doing the sorts of things where a 6-10 year old CPU is still fine, then you probably don't need to upgrade the machine at all: my mother was quite happily using a desktop of that sort of vintage, with no upgrades, until she replaced it with a laptop last year.
For reference, the machines I use when I need a bit more processing power than my laptop have dual (ZFS, mirrored) 3TB disks, 512GB SSDs split between log and cache device and 256GB of RAM. The large log devices speed up write performance, because you're almost always doing sustained linear writes to the spinning rust. The 256GB of RAM means that you very rarely even hit the SSD for loading files. They have 24 cores, and I can very easily saturate them all. If you gave me a 48 core machine, I'd use that instead, but currently the extra performance isn't worth the cost (doubling the number of cores roughly halves the time it takes for various things, but the linear gain is much smaller - going from one hour to half an hour was a big win, so was to quarter of an hour. Going from three minutes to one and a half minutes isn't that exciting).