Have you looked at Hasura for the second question (graph frontend + relational backend)? That's basically GraphQL on top of Postgres.

As for the first question - I've tried using Neo4j and ArangoDB for relatively large-scale graph querying (1-2TB of data) and both couldn't hold a candle to Postgres or MySQL in terms of query performance for cost. Neo requires you to store most of your data in memory and Arango isn't great for cross-shard querying.

Unless there's some major new graph DB that comes out in the next few years I would still bet on relational being dominant in 2030.