I mean, you’re right about the rich getting richer being a problem (and the citizen/land problem, though only relevant in the Late Republic, not the Empire), but wars were a net drain on the Empire’s finances. A few men got very wealthy from them, but they were largely unessential to the Empire’s economy and tax base.

A lot of the problem was not only a declining tax base from increasing mismanagement and unchecked barbarian incursions in the 3rd century AD, but skyrocketing costs from the Emperor’s court, which slowly discarded the previous power-sharing arrangements with locals and Senatorial elites in exchange for a more monarchial system in which the Emperor controlled access to all the levers of power, funding unspeakable luxuries for the ‘majesty’ of the office, and a massive army of incredibly corrupt bureaucrats with very little actual power, who nonetheless made themselves indispensable by eliminating all alternative forms of running the Empire.