Taking the OP’s question literally: "Are cable “upgrades” just as likely to make your system sound worse?" the answer is a solid "NO!" They are not just as likely to make your system sound worse." In fact, it is highly UNLIKELY that they will make your system sound worse.
To quote William Low: "Cables can’t make a system sound better. They can only make it sound worse. It’s all about damage control." My take here is that the "perfect" cable would be NO cable. Anything in the signal path will degrade the sound to some degree. We’re just trying to minimize how (badly) they mpact the sound.
To address those who do not feel that cables make a difference, I would submit that it is impossible for 2 cables with different materials, geometry, gauge, dialectric (insulation), shielding, termination method, termination, etc. to sound the same. I’m not asserting that there will be a sledgehammer difference in those cables, but there will be "some" difference(s).
Comparing cables:
One attribute of cables can be dynamic compression. They simply hold back the energy presented to them. When "better stuff" is put in the signal path, dynamics can get more pronounced (louder). This may, or may not, be a good thing depending on the system -- and, the listener. "Opening up the top end" sounds like a great idea. Except when it reveals "warts" in the signal path -- or issues with the room acoustics. Or, the changes are simply not appealing to the listener. So, we decide to "dumb it back down" because the cable is "too bright" or "too forward", etc. I’m not saying that this IS the case in every system, but it could be the case in some.
As some have suggested, cables need burn in time. So, your first (negative) response to a cable may be a bit premature. Patience.
Yes, upgrades can definitely create "mission creep", and present the perfect storm in our pursuit of better sound.