You seem to be setting up a divide between armored troops and firearms in the question, assuming that adopting the latter means disbanding the former. While any weapon and armor technology always exist in a state of tension, armored soldiers continue to be used alongside firearms up to the present day.

The adoption of firearms and the decline in personal armor are trends that don't quite overlap. The period from the 14th-16th centuries is not only when firearms and firearm tactics were adopted and refined into a coherent tactical system, but also saw the peak of armor production, sophistication, and use. Firearms delivered a powerful blow to even heavily armored soldiers, but a blow comparable to certain heavy crossbows that had existed alongside armor for centuries.