Just to further illustrate the point, I just saw this comparison of the Mets lineup now vs. one on July 23. 5 of the 8 (pitcher excluded) players in the lineup tonight were not in the lineup in July. Sure it's not the only reason the Mets are rolling right now, but it's a big part. Their hot streak began with a scorching August (20 wins). I don't know the specifics as to the changes the Cubs or other teams have made during the same time frame though, maybe all teams are on another level right now but I think the Mets pitching has been there all season, but they were still losing games due to low run support. But since August the batting has caught up.
July 23
Granderson, RF
Tejada, SS
Flores, 2B
Mayberry, LF
Campbell, 3B
Duda, 1B
Lagares, CF
Recker, C
Colon, P
Oct 21 (game 4)
Granderson, RF
Wright, 3B (was injured)
Murphy, 2B (?, maybe was a night off)
Cespedes, CF (still with Detroit)
Duda, 1B
d'Arnaud, C (was injured)
Conforto, LF (still in minors)
Flores, SS
Matz, P