I could not agree more. There are certainly lots of snake oil products in the audio world, but cables (power, interconnects, and speaker) are not things I consider to be snake oil. In my last round of upgrades, the sales guy I've used for 20+ years (who I consider a friend) was over doing some tuning on the system one night while we had some pizza and beers. After making all the changes, he said "I'm going to suggest one more upgrade, and it won't cost a penny." My wife was very glad to hear that last part! I had been using a power conditioner on my system for years, both to condition the power for the source components, and to act as insurance for power surges. The upgrade was plugging the system straight into the wall, rather than going through the conditioner. The difference in the sound was clearly audible, and in fact much better.
This lead me to install a dedicated circuit just for the audio gear, straight from the panel. It also taught me that power draw is something to be paid attention to, in particular if you listen to music with a high dynamic range. When the system needs power, if it's not there you will hear the effects, so although I didn't buy any special cables (what came with the equipment is plenty good enough) it illustrates the point that all parts of the system need to be at least looked at.
But as you say, it will depend a great deal on the system. If your system is very muddy to begin with, these thigs likely won't make much difference. My current system is extremely transparent, so small changes can easily be heard. As for something digital being "all the same" well if you haven't heard how much difference a DAC can make in the sound of the same source material, it's something that should be experienced.
Any good shop will let you demo equipment in your own home, with your own sysrtem. I've done it many times with speaers, amps, sources, and the best component for your system is not necessarily the most expensive. If the system works together well is the key. Finding the right mix can take time, but when you get there it's pure musical enjoyment.
Cheers, Al