You have to remember impeachment is not the same as the kind of annulment that was done on Formosus. Impeachment just removes you from office from the time you are impeached. Annulment retroactively declares that you were never officially in office and invalidates all of the actions you took while in office.
To put it in modern American terms, suppose Congress impeached Barack Obama next month and removed him from office. Obama would no longer be President but the Affordable Care Act will still be in effect because he signed it back in 2010.
Now suppose that Jeb Bush gets elected in 2016 and there’s a strong Republican majority in Congress and a Republican majority on the Supreme Court. And in 2017 they get together and decide that the Birthers were right all along - Obama wasn’t born in America and was constitutionally ineligible to be elected President back in 2008. So they declare that his signature on the ACA in 2010 held no legal validity and the ACA is not a law.
More breaking news!!! DNA confirms he did father a child. Surely something has to be done about this. Time to amend the constitution to allow retroactive impeachment proceedings so congress can pole into his sex life.