I'm not the OP, but thinking well beyond the original topic of in-memory reactive programming (where my answer would be quite different) to the world of long-lived durable databases... one perspective to consider is that any system built around N-ary relations can automatically benefit from the full range of relational algebra for transforming and composing both base data and derived relations. The flexibility of N-ary relations is largely what has kept SQL databases relevant despite the flaws of SQL itself.

In contrast, a system that only handles base data in terms of triples assumes that you have a perfect attribute-oriented information model figured out upfront. But given this is rarely the case users will want tools that help them to easily transform/migrate their data and schema over time. Ideally this takes the form of a declarative language that minimises the amount of code that needs to be written. However, without a compelling end-to-end transformation language figured out I think any alternative database systems with their alternative information models (triple-based or otherwise) will struggle to compare favourably with mainstream databases, where declarative data munging with SQL is considered valuable and routine.

Triples may well prove to be the best way to handle information in software over the long-term, but I'm not sure that the systems which currently work with triples are good enough or widespread enough to test that theory.