In the finale, Walt tells Skyler that after that night, Jack's gang will no longer be a threat to her or the rest of the family. However, Walt's final plan at the end is a Hail Mary. He probably entered Jack's compound knowing that this would likely not unfold as planned. If Jack's gang wasn't wiped out, how would Skyler's safety be assured? Also, it was established that Jack's gang had members that were imprisoned. Could these members potentially harm the White family as revenge for Walt's actions?
If Jack's gang wasn't wiped out, how would Skyler's safety be assured?
There were only two possible outcomes: Either the plan succeeds and Jack's gang is wiped out, or the plan fails and Walt dies (or is taken prisoner like Jesse).
If Jack's gang is wiped out, it's clear why the threat ceases.
If Walt dies, the threat will also cease as there's no reason to go after a dead man's family anymore. If Walt is taken prisoner, his "cooperation" will be ensures by not harming his family and threatening to do so if he does not cooperate (similar to how Jesse is being coerced by threats about Brock).
In all outcomes, Skylar is safe. In the single outcome where Walt is imprisoned, it would still be better to let Skylar believe she is safe.
Also, it was established that Jack's gang had members that were imprisoned. Could these members potentially harm the White family as revenge for Walt's actions?
Who precisely would tell the tale of who killed the gang (not in prison), if the gang is wiped out? How would they know for a fact who did it?
On top of that, would lone members be able to retaliate in any organized fashion?
On top of that, would they want to? There's no gain to it (it's not bringing the gang back from the dead), and there's a lot to lose on it (reincarceration, death).
Breaking Bad makes an effort to showcase that criminals are humans too. It's unlikely for any crime to take place "just because". Killing or harming Skylar would have no justification other than "because we can", which is not thematically compatible with Breaking Bad's perspective on crime.