Even in levels with nuke solutions, it should generally be possible to preserve any replay simply by delaying the nuke assignment by 5 seconds.
The replays are not the big problem. The levels are.
I know quite a few levels where the timer on the nuke is essential to the level in combination with the mechanic that one lemmings after the other gets it.
Examples:Timer needed:Armani's Supernova
Crane's Are you kidding me?!
Deceit's Enter at your own risk....
One of my own levels from United
...
Only the fact that one lemming after the other gets nuked is essential:Basically all other remaining nuke solutions.
The timer is needed in the levels in the first category in the sense that after the nuke button has been pressed the lemmings get assigned the timer chronologically and still walk for the 5 seconds giving them time to still reach the exit. This can be used to let a lemming explode with the nuke, create a gap in the assign chain by letting the next lemmings exit and the initial explosion allows the rest to be saved from the nuke.
So the delay between the first lemming exploding and the last ones is being used.
If NL was a new engine the deceision would be easier. But after so many years I think it's not worth the cost to remove this. You essentially just break a bunch of levels for no real gain other than a slight code clean up. Also, even if I'm usually not a big nostalgia guy, having a count down before the big final firework is classy and neat.
So, I say no here.