Although Firehawk did definitely have its problems, I think it could've been possible to keep that coaster running for a bit longer if the park really wanted to, but when the opportunity came up to put in a giga coaster, it was a pretty clear decision for the park to replace a problematic coaster with a more reliable and popular attraction.
I think the "reached the end of its service life" reasoning definitely applied more to Vortex than it did for Firehawk. I'm certain Kings Island would've preferred to keep Vortex in operation until a replacement was ready for construction. It seems the problems with Vortex were significant enough for Kings Island to pull the plug on it well before such a replacement was even conceived. Unfortunately, I think the problems with Vortex just came at the worst possible time. Not just because Firehawk's replacement was still under construction, meaning it would take at least another three years before Kings Island's next major coaster, but also because of the pandemic which delayed that project even further until we're still talking about what could go into that plot five years later.