there are positions and times when defense certainly matters, especially at SS and 2nd; but a good offense always has been, and always will be far more important.
There's only a few truly stand out defensive wizards in any given generation, it seems, as much of solid, necessary defensive play is pretty much standard at the ML level. By the time you're playing at ML level, you should be able to do 95% of what is required in defense.
There are instances where a single defensive play can indeed change a game, but statistically, it's nothing like how important offense is.
I think it was one of those James stats from a decade or so ago, showing that overall, defense is an insignificant factor in determining a team's success from year to year. People want to believe it matters, because they feel they see things all the time. it's just that the reality doesn't exist in the numbers.
When teams lose because of bad defense--that's generally because of errors. Not so much bad defense, really, just bonehead mistakes. Anyone can make those.