Subs can spot and threaten ships with torpedoes, and diving can help them avoid some threats that make life harder for destroyers. So what keeps a skilled sub from having the same influence as a skilled DD? Is it mainly about how much they can affect objectives and the fight around them?
Surviving longer and landing damage are not the same as changing the match. A sub can threaten a target or gather information, but that value depends on whether it affects what the other team can safely do. If it spends a long time getting into position, or stays away from the areas where the teams are contesting space, its influence may be narrow even when its play is careful.
That makes the comparison less about whether a sub can do some DD jobs and more about how consistently it can turn those jobs into pressure on the battle.
I think the safety argument can get overstated. Avoiding a threat is useful, but it does not automatically create pressure elsewhere. A DD that is present around an objective can make the enemy account for it even without scoring a lot of damage. A sub may be better off staying alive, but if that means it is not affecting the current fight, the team may not get much from that extra safety.
“Least impactful” sounds too absolute to me. A strong sub can still make a real difference through spotting and torpedo pressure. The catch is that those contributions can be very dependent on position and opportunity, so one good play may matter a lot while the sub has fewer ways to apply pressure across the whole match. A skilled player raises the ceiling, but does not make every situation equally useful.
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