[throwaway4233]

Based on the way you have described their API, I can make 2 assumptions

1. The might have had well supported API until the point at which they realized they wanted to have more people to use the webapp, rather than having the data sent through an API. More time spent on the Hubspot webapp means, more chances the customer notices the other services Hubspot provides and pays for them.

2. The versioning APIs that you see are most likely due to an enterprise deal Hubspot made with a large company who asked for variations in the API, and Hubspot introduced a v2/v3/v4 instead of modifying v1 (to not cause disruptions for their customers using v1).