A couple of points. The Romans couldn’t keep the republic. They got an empire when they started punishing their political opponents after they got out of office (that was the real reason behind Caesar’s becoming emperor). The church (both the Catholic and the Orthodox variations) didn’t support democracy. They were just fine with tyranny as long as they had a seat at the table. As to Africa and Islam, to hell with both of them.
Rome’s largest failure was social decay and straying away from what made them successful. They matured and feasted in their immoral behavior to the point corruption moved in and the collapse came.
They ignore the points that made them rome, much is what the G7 is doing through liberal immigration policies. Look at the UK, germany , france all faltering now as we speak.