If there was always a positive effect on SQ going forward, then we wouldn't have such a robust 2nd hand market for used gear :P
When you don't like gear at first, and later you do, it's often attributed to burn-in. When you do like gear at first, and later you don't, it seems you were just "ready to try something different".
I'm not saying that burn-in works in reverse, it's just that I think the whole thing is way overblown when it comes to electronics. I had one tech tell me "these caps will sound better with burn-in" after he'd wired them so egregiously WRONG that the amp had a 6dB/octave low frequency rolloff starting all the way up at 1kHz!! Nope, I really don't think burn-in is gonna help much there...
For what it's worth I do believe (and have observed) that break-in can be significant with new speakers & headphones, and to a lesser extent cartridges. It may exist for electronics too, but it's far less significant in my experience.