People who seen death is the only one who can see thestrals but how can the other ride on it then??

Imagine you are in a pitch black room and you come across a chair. Even though you can't see the chair, you can still note its physical presence with your hands.

Just because they couldn't see them didn't mean that the thestrals didn't exist for them. They were still tangible objects, but not visible for anyone except Neville, Harry, and Luna at that moment. Hope that's not too confusing.

If you're riding one with someone who can see them, you're alright then.

You can touch a thestral even if you can't see it, if that what you meant

See and feel are not the same thing. Nobody ever thought of feeling the thestrals because they had no idea they were there - except once you see them.

Thestrals symbolize (IMHO) the conscience one can have of death.

When Harry, Ron, Hermione, Neville Luna and Ginny are riding thestrals to London, true only Neville, Harry and Luna can see them, but Ginny, Hermione and Ron obviously still ride them. It's just that they can't see them. They can still feel them though.

Just because you can't see something doesn't mean it's not there.

sa pamamagitan ng this same logic, blind people wouldn't feel anything, cause they can't see stuff, right? Uh huh.

They couldn't see the thestrals, but that didn't make them any less real. It probably made them harder to ride, sure, but not impossible.