Wow, that's an impressive strawman.
No one expects Forbes to be a defensive stopper. If he were average on defense or even below-average-but-competes at even a Matt Bonner level, that'd be great.
But multiple advanced stats literally find Forbes to be the worst defender in the NBA. And, again, it's not even his fault, really. He's usually in the right position and he usually knows what he's supposed to be doing. The problem is his lack of size or length combined with a lack of athleticism makes it impossible to competently compete on the defensive end against other starters.