There was really right idea to introduce Spring Integration own 'integrationConversionService'. We did not want to assume that the ConversionService used for dependency injection and bean creation within the container itself would necessarily use the same Converters that are needed for integration flows at runtime. They really are two different concerns.
The rationale for not falling-back is the same; we create a new DefaultConversionService instead of using one that might have been configured explicitly for use within the BeanFactory. The main idea is that someone may register Converters that are intended for use when wiring Bean constructor-args and properties that would actually produce unintended consequences if used at runtime for Spring Integration expression-evaluation against Messages, etc.
If on the other hand, someone DOES want to use the BeanFactory's ConversionService for Spring Integration also, they can simply register one line of config with: