I notice the article says that US had destroyed only about 1/3 of [missiles and stuff], but as to the second third? Also destroyed.

Traditionally we call that "2/3", but what do I know of statistics and reporting?

Why would they send such an old plane into a combat situation unless they wanted it to get shot down?

F-15s are appropriate for many missions. I think you are thinking of the chapter less capable F-16s instead?

The F-15EX Eagle II entered service in 2011 and is still being built.

As to why: payload capacity.