Learn English – Word for someone who receives commission

Plastic fan (or just plastic) fits your description, but it tends to be a derogatory term. Here is the Urban Dictionary entry:

A person who roots for a Non-Hometown team based on that teams good record.

You can call this person a day late and a dollar short.

This is different from hindsight, which is the ability to describe things in the past and only bring them up in the future.

A person who is a day late and a dollar short may certainly have been aware of some important fact prior to or during the time when sharing that information would be helpful. But the fact that the person doesn't share makes their offering the information after the fact essentially useless.

A person with hindsight (or someone who is a Monday morning quarterback) doesn't necessarily know anything useful when it matters. That person is only able to pretend to be knowledgeable and aware afterwards.

A more literal, less idiomatic phrase might be too little, too late.

Here is an example of a day late and a dollar short in use:

I knew you would get stuck on the sand bar at low tide when you told me you were arriving this afternoon.

Well, as usual, you are a day late and a dollar short. It would have helped a lot if you had told us that when we were talking to you this morning.