You really do not to watch and read more.
What you say is absolutely nothing new within the genre's of Assassins and Ninja's. NOTHING.
Many start with a big infiltration and a big kill of an important and often highly guarded character. The Assassin or Ninja gets in, does his job and the kill or many kills are accomplished.
There are also all sorts of Special Opp's books that do similar. A tactical unit gets in, pulls off a big kill or regime change and gets out.
Never, NOT A SINGLE TIME, does it end up in a /book moment where it makes no sense to go forward because you should all just assume they could do the same thing against each and every time.
I cannot help you Clippy if you ar4e completely ignorant to Assassin, Ninja or Special Forces genres and that leads you to wrong conclusions that therefore it is /book, /movie, /tc show. You are just factually wrong nad until you educate yourself you will continue to repeat that ignorance that you are not.