Creating a ES6 class for every API endpoint's response
Hard coding outside data source should be avoided.
Hard coding outside data sources will present an ongoing workload. This will be compounded by the fact there is no validation of data.
At anytime the data source may change, add, or remove properties. You will not know about the change (if you don't personally follow the source doc feed) until it fails or a customer/consumer reports the problem, or worse just stops using it.
Testing for a valid response will not protect against modified data.
Even if you validate the data it will just improve response time for the fix / update but not the underlying problem.
The cost.
Every change will require an update, test, documentation, deploy, monitor, release, and pray cycle for the life of the service. As a business I would have to pass this cost/ risk on to the client or revenue source, and having a programmer on call is very expensive. In other words this type of design can kill a service.
For the sake of auto complete it is not worth it.
You are best of generating something that Vs2017 can read (I don't use it so you will have to look in the docs) and hard coding as little as possible that is outside your direct control.