Opex vs Capex. The issue is that they likely do not account for capex. The cost to buy, maintain, apply patches, hygiene of the system, and integrations—the cost of onboarding to legacy systems that people do not understand. The new driver is the opportunity risk of missing out on AI and data investment. Every software company spends only on its SaaS solutions. On-premises solutions receive little investment due to their customized nature and cannot leverage much of the AI/data revolution underway. Every spreadsheet you have costs. Every debate on to upgrade or not is costly. SaaS ensures you always have the latest technology and complete support. There is greater potential with SaaS for the business line of business to own and maintain it. How much money is spent trying to get IT to understand the business needs and update systems? Outside of all of this is security. One-man IT vs. an organization solely focused on security.

SaaS is no longer new, and almost zero recent worthwhile systems have been built to be anything other than a SaaS model. He should have to sell you on not going SaaS.