Does this actually pass as an academic study? It is presented in the format of a study (it looks like one) but to suggest GraphQL is better than REST based on the expenditure of effort for building out the first iteration of code for cherry picked scenarios is a first-year undergraduate CS101 study at best. Also, who funded this study? I want to know the actual costs of GraphQL. What are the tradeoffs with REST? What…
Also from what I've seen the approaches that may result in single ideal query plans in your database increase the complexity of GraphQL significantly with "GraphQL to SQL" compilers which still may not give tuned SQL especially with large graphs. Reminds me of using ORM's where often the generated SQL wasn't performant/slow. Even if it does work the complexity of your solution just increased - all for what? To translate one query language into another? There's also times where it may pay NOT to expose too much of your internal domain structure to public API's which I feel from a naive developer's perspective GraphQL could encourage (direct internal API structure to contract mappings).
It feels, at least to me, that it is another abstraction layer that solves a problem that could also be solved at the org level. If there's suddenly a use case that requires a tuned query/algorithm/index as well (happens a lot from my experience) then its easy to add as a separate endpoint and test for regression test against other endpoints/use cases supported.