Why are you comparing it to Saddam's WMD? You think US can invade a country because it shot down a civilian plane, that too full of its own citizens? It's not just the US that's saying this, countries that haven't even supported the US in this Iran-US mess are also claiming the same.
It has nothing to do with invading Iran. It is about how US controls the media and spreads propaganda.
Why would they bother?
1.It hugely destroys Irans reputation among Europeans, justifies Trumps sanctions,
2.Oroves to the Europeans and all anti Trumps that Trump was right about Iran all along.
3.Stops all further Iranian attacks in Iraq, because after this, Iran had the morale highground after Trump assasinated Soleimani, Now Iran lost that morale highground, if we do anything in Iran, we will be seen as an aggressive country that has to be stopped.
This is huge benefit for Trump