So, sanctions are not intended to be imposed in perpetuity – if a party stops the activities that resulted in their being sanctioned, they should be able to get de-listed.
Well… so, if you’re a firm that provides military aircraft to the Government of Syria, or a militia, etc…. and then the Syrian civil war finally ends with Assad prevailing over the remaining forces opposing his rule. So, you’re no longing providing the aircraft because there is no longer a need…
Can you get delisted? You didn’t really change your actions through a change of heart – but you’re no longer doing the thing that got you sanctioned.
Imagine the calculus for a firm who believes that Assad will win the war in the next year or so. Do you take your chances, given that you can say “it was only once – I’m not doing it anymore”?