The only reason an assault would need to be accomplished within a time limit is because of imminent threat of a strong counter-attack or changing battlefield conditions (e.g., nightfall). Lockdown ticks one box in that there needs to be a sense of urgency, levied with a measurable mechanical effect, i.e., the game ending, but it utterly fails to embrace why the attackers need to motivate. Argue the whys and wherefores of players refusing to de-camp and get a move on all you like, but there is no reason at all why the game needs to be ended a) so quickly and b) in such an awkward an unsatisfying manner, as is the case with the current Lockdown mechanic.
One way to represent Lockdown more immersively might be to steadily increase the time between reinforcement waves for the attackers, and have this mechanic kick in at the same time Lockdown does now. The important thing to add to this, to instil a sense of urgency on the side of the attackers, would be clear on-screen (and perhaps accompanying audio) warnings that the assault is in serious danger of losing momentum. To add to the threat of a broken assault the defenders could also receive some form of indication that attacking forces are beginning to thin thus may trigger them into mounting a counter-attack. To perhaps bring back a little taste of the gloriously epic struggles that took place in RO1 from time to time, have the attacker reinforcement intervals return to normal as soon as the defenders manage to take back a territory. (Naturally, the game would end when there are no more reinforcements, or the defenders retake the first territory on the map, or the attackers claim the map for themselves in its entirety.)
Other people will no doubt have better solutions but as long as the current deeply unsatisfying and horrendously arbitrary Lockdown system remains in place RO2 battles will continue to end too quickly and unsatisfactorily, for either side.
For what it’s worth, I don’t believe that increasing the time remaining on the current Lockdown system will make the blindest bit of difference to a steadfastly intransigent attacking force; if they won’t get off their arses now then they won’t knowing that they have even more time to stay put.
TL,DR: The bottom line is irrefutable, and it’s that the current application of the Lockdown concept is deeply unpopular so something needs to be done about that.