In our specific case, I’d say it depends. Right now we have been in the lucky situation where we would not need an app migration framework. Relevant data is stored in entity properties - so simple export-import should capture those.
One problem we noticed though is that Server and Cloud implementations are drifting apart and this may make an app migration framework necessary for us in the future too. Specifically, indexing array entity properties stopped working in Jira Server 8.x which forced us the change the data format in the Jira Server implementation and therefore making migration more difficult.
Ultimately, I’d say an app migration framework is for the benefit of customers and vendors if it can account for the different requirements from most vendors.