And where is the evidence for your theory?
US and Israel have a tacit understanding with Russia that Assad-led regime won't be toppled. However, this understanding is not written in stone. US can decide to topple Assad-led regime on any given day and Russia can do nothing about it.
Also, you are terribly mistaken if you think that US and Israel take Russia on-board for any operation they conduct in Syria. They only issue a warning to Russia in advance that we are about to strike that part in the region so make sure that your troops and/or assets are not there. In-fact, even this level of cooperation is not always a given.
Just accept it! S-400 makes little difference against military might of US and Israel. They have the relevant countermeasures for it and should it ever come down to that, you will see that S-400 system blown to bits during the opening salvo of the operation.
Their is hell of a difference between fanboy ravings here and ground realities our there. Russian assets sound good on paper but US has a vastly bigger economy, R&D and Industrial base to prepare an appropriate counter for them.