Every human is disposable in a Transformers movie.. some more so than others (i think John Turturro was hilarious in them).
They got rid of Fox and replaced them with an equal hottie and no one batted an eye or started a "Bring back Megan!" campaign.. the same applies to Shia. As Fandango said it's inexplicable how he keeps scoring these blockbuster roles but then there's actor's who are just Hollywood darlings who score some big movies because they once starred in the first one which was successful and execs now think it's because of them.
The movies are about giant robots beating the crap out of each other.. humans are just collateral damage.