Note that the defending side has the obvious advantage. The supplies can return to the system after getting blown up, the attackers can't. When the shield is down for just a minute, a supplier can refuel the shield and extend the whole operation for hours longer.
Even the most active faction can NOT siege a base 24/7 without making a mistake. The defenders can make plenty of mistakes before the boom, the attackers can't or it's all for nothing.
