This is a good point. At the same time, bacteria have been mutating for much longer than birds or land animals, and they iterate generationally at an unbelievable speed compared to their multi-cellular cousins.

The risks should be taken seriously, even the seemingly far-fetched ones, but need to be viewed in the context of an already cut-throat competitive landscape. IMHO, the bigger risk is intentionally designing malicious life forms (bio-weapons) more so than than beneficial ones getting out of hand.