As Asya pointed out in a comment on SERVER-23313:

I don't think this error message is always correct/helpful:

"errmsg" : " 'newRoot' argument to $replaceRoot stage must evaluate to a valid Value, try ensuring that your field path(s) exist by prepending a $match: {<path>: $exists} aggregation stage.",

If the field (say "$a") is not an object we correctly say

try ensuring that it evaluates to an object by prepending a $match: {<path>: {$type: 'object'}}

Is there a reason not to always say that even if the field is missing? Also if path is missing or there but null they don't need to match, they might need to $ifNull inside newRoot, maybe to preserve document counts or fill in some default values.

Also they might correct such errors via checking $type of path (in addition or instead of $ifNull) to fill in default values...

We should take some time to ensure these error messages are clear and helpful, without proposing a solution that might not always be ideal.