The discussion on cannon also assumes that cannon were developed and that there is no magical counter. Early uses of gunpowder were more flash than bang and were more effective at instilling fear than actual damage. When you can do cantrips that are just as flashy that actually do damage, not to mention even first level spells, who knows whether or not those early weapons would have amounted to anything.
In addition, for every weapon of war there is typically a counter. We don't have spells that counter cannon in D&D because it's not a core assumption.
In addition, for every weapon of war there is typically a counter. We don't have spells that counter cannon in D&D because it's not a core assumption.