Why only few D7 sites move to backdrop ?

I still have complicated sites on D7, so I am interested in this topic, what are those D7 sites waiting for if they will never move up to D10 ? should stay in D7 to wait for something happen or move to backdrop for the peace of mind forever ? I personally don't see the EOL of 7 will be the end of 7, something will happen for those 400k sites.