Not sure if you are more agreeing or disagreeing with GP, but cost is not just computing cost, but also complexity and the higher risk to reliability that comes with it.
JSON-RPC makes sense if one only considers the abstract case with a single client and a single server box and minimal networking. Technically it's a viable solution but just not practical.
HTTP on the other hand is nice as it separates the business logic stuff like the RPC parameters from stuff that's interesting to the transport-network-thingies that sit between you and the data on a server, like load balancers, DoS protection, authz, rate limiters, ... etc.
It's a trade-off. Mostly pure REST is nicer for external APIs where it's just more important to follow the principle of the least amount of surprise for the ed user. For better or worse, people kind of know how to deal with REST clients.
Graphql, SOAP, etc. all force the user into doing complicated stuff with libraries, figuring out parsing for their techstack, etc. This is fine for internal microservices where you might not care too much about this and can standardize things a bit. But for external users it's more challenging.
With simple Rpc stacks (like JsonRpc), basically the up side is you get to rely on code generation to deal with networking. Cutting down on the amount of ceremony for adding new stuff is definitely nice in a fast moving team. But I would not use it for external APIs. It's kind of a legacy way of doing thing. An alternative to something called XmlRpc, which used to be a light weight alternative to SOAP when people were still doing XML and when MS came up with the genius idea to stuff something called an XMLHttpRequest in Internet Explorer. This was what enabled Ajax websites early on. The X in ajax of course stands for XML. That was just before json got really popular and people started getting really pedantic about their REST verbs.
Anyway, short history lesson. I'd not over think this and indeed simply use REST for public/external APIs and use whatever works for you internally for e.g. client server traffic between your browser/mobile apps and your servers or intra microservice communication. If it's internal, you control the techstack and you can sacrifice some things for making it faster/easier to develop and use without too much regard for what the other side is going to be using. Because you control both sides. REST APIs can be a bit of a burden to maintain. Lots of boiler plate client and server side. Not always the best choice internally.
JSON-RPC makes sense if one only considers the abstract case with a single client and a single server box and minimal networking. Technically it's a viable solution but just not practical.
HTTP on the other hand is nice as it separates the business logic stuff like the RPC parameters from stuff that's interesting to the transport-network-thingies that sit between you and the data on a server, like load balancers, DoS protection, authz, rate limiters, ... etc.