> JSON schema exist exactly for that reason, hacks like adding type is just nonsense

Creating schemas for declaring types is literally a hack for adding type! I think its very bizarre to say "You dont need to do X because you can just do X."

> Also how hard is to write {"type": "datetime", "value": "2021-02-23 12:46:37.07"}

"datetime" isn't a universal format specification. 2021-01-10 could be October 1 or January 10. What time zone is this in? Local to the sender, UTC, local to the recipient, somewhere else? Is this a 24 hour clock or something that gets a modifier for the other half of the day? You're making some classic mistakes that people make when they don't think about the complexity of the problem domain of communicating information unambiguously.