Whoever gained the most points in the past 3 to 5 minutes (bonus points for squad activities excluded) ought to be promoted to be the Squad Leader.
However, to smooth it out, this should only apply if the difference in score gain (mind, not total score, so newcomers have a chance) is greater than 20% to that of the current leader. That way, severely underperforming leaders will be demoted to their rightful place and actively engaging in the fight will be promoted.
Absolutely no one needs that "Squad Leader" sitting in the back and sniping next to the spawn beacon at the edge of the map.