Trying to copy, erasure, steal, or interfere with a skill in any way is the last thing you should try against a Tensura character.
Shuna has a unique skill and normally no character can copy a unique skill, it's against the system, so you have to interfere with the system to copy a unique skill. It is clear that Kars cannot interfere with the system at the level of a unique skill user. You say that the skill in question will be seen, understood and imitated in the future, but this is even more impossible. The system, the source of all skills, governs all laws of time, causality, fate, logical contradictions, time, causality, fate, time, causality, fate, in the tensura multiverse, which contains an infinite number of branching timelines and countless different worlds. Even understanding the use of magic, the most basic level of the system, requires extraordinary genius. Yet you say Kars can deny the system and understand and copy Shuna's unique skill. Only Ultimate Skill users are not bound to the system, and only Ciel, the Ultimate Skill of Rimuru Tempest, can literally control the system. Even if Kars tries to do so, the system will appear and resolve the logical contradictions.