1) Are there shared traits of sequences that become widely professionally played?
2) Are there shared traits of sequences that cease being widely professionally played?
3) ???
Speculatively, I assume (1) happens when a top player adopts a line. Everyone who wants to beat that player studies it. Because they have studied that line, they also play it (or variations upon it).
And (2) happens once an equitable (or biased) set of "best" continuations emerges.
One might predict that popular sequences' continuations will have more (and lots of) variety, where declining popularity sequences would have less.