Atlassian has some guidance on the licensing policies for migrations here: Transfer your self-hosted license to cloud
See the section entitled "App licenses". The policy for app licenses is that there is no transfer or credit from Server/Data Center towards a Cloud license (sorry).
Application vendors rely on Atlassian for licensing and purchasing, for which Atlassian charges a percentage of the license fee. We are always happy to hear feedback on licensing policies but be aware that they are set and controlled by Atlassian and that feedback given directly to them about their policies is probably the best way to get it heard by the folks who set the policies.