I can't lie that I, like many others, use the term incorrectly. Or rather the definition it has seemingly become.
When the first MacGuffin was coined by Hitchcock it was used to refer to a plot device that kick-started the movie but ultimately fell by the wayside as the movie progressed and could be replaced by any number of things.
However over the years MacGuffin has been changed to mean, literally any important plot device in the movie either a character or object. So stuff like the Arc of the Covenant, the Infinity Stones, the Death Star plans, and so on were suddenly being classified as MacGuffins despite them all being important to the movie all the way through and not something that could be replaced.
Honestly I feel like we should go back to the original definition because quickly people have begun to label everything a MacGuffin and if everything is a MacGuffin, doesn't that make nothing a MacGuffin?