This strengthens by providing a mechanism by which the increased prescription of new antibiotics could cause drug-resistant bacterial disease outbreaks, by reducing the competition for drug-resistant bacteria due to killing a wider variety of other bacteria.

This is irrelevant. The argument only claims that newer antibiotics will displace the older penicillin, regardless of how long penicillin has been in use. This also doesn’t tell us anything about how penicillin has affected antibiotic resistance so far.

If anything, this weakens the argument by rebutting the author’s claim that drug manufacturers would only sell newer antibiotics—after all, if penicillin became more profitable, they’d probably put it back on the market. Either way, this doesn’t explain the antibiotic resistance.

This is irrelevant, because the penicillin shortage predicted by the author is a result of what products drug manufacturers choose to sell, not the cost of treatment down the line.

This weakens the argument, because it demonstrates how new antibiotics could actually reduce antibiotic resistance, by killing bacteria that resist penicillin. It certainly doesn’t strengthen the conclusion that new antibiotics would lead to more resistance.