Artemis is big, but NASA is much more than that. If a Flagship mission says "no thanks", that's a serious customer speaking directly. EC is about the same size as the current Artemis program.
EC cost estimates are currently about 4-5 billion for the whole program while a single SLS launch is ~2 billion.
The whole SLS/Orion program runs at few billion each year and it's already spent more than 10x the entire cost of Europa Clipper.