When I'd choose React over WordPress

For a site that is mostly content e.g. pages, blog posts, SEO, and updates the client can manage themselves.

Budget and timeline suit a proven CMS with themes and plugins, and you don't need much custom business logic.

For sites where users log in, see different data by role, or work through a flow: dashboards, portals, booking, internal tools.

You need tight API integration, real-time updates, or behaviour that plugins can't easily provide.

Agencies often hit the point where a client asks for a small portal or a bit of automation on top of a marketing site.

I'd keep WordPress for the marketing site and build the product bit as its own app when it gets complex enough.