Looking at the NL data from 1991 you presented I don’t see were triples starting declining since 1991. That year the league hit 441 triples and stayed in the 450-500 range up until about 2017. To me that’s when the decline began.
I'm talking about the proportion of extra base hits that were triples, not the raw numbers of triples themselves. Expansion in 1993 and 1998 messes things up if you're looking at the raw number.
Looking at the NL data from 1991 you presented I don’t see were triples starting declining since 1991. That year the league hit 441 triples and stayed in the 450-500 range up until about 2017. To me that’s when the decline began.
I'm talking about the proportion of extra base hits that were triples, not the raw numbers of triples themselves. Expansion in 1993 and 1998 messes things up if you're looking at the raw number.