What?? That's an insane argument. That's like saying if one client sets column X to 1 and another client concurrently sets SET y = 2, one client's writes will be LOST. It shouldn't, and it doesn't. If it did, nobody would use Postgres. This issue only exists with PG's JSON impl.
What?? That’s an insane way to describe what I’m talking about. Data/transaction isolation is very complex and extremely specific to every use case, which is why database engines worth anything let you describe to them what your needs are. Hence why when one client writes to Y they specify what they think X should be if relevant and get notified to try again if the assumptions are wrong. An advantage of specifying your data and transaction model up front is that it will surface these subtle issues to you before they destructively lose important information in an unrecoverable manner.