$?="" is an input. It doesn't work like $iif. $iif (and IF for that matter) compare 2 values and do something based on whether or not the comparison is true or false. $?="" just gets a value. They are completely different things.
You're looking at it only with null and saying that $iif($1 != $null,$1,some default value) would be the same as $1=some default value. How then would you write that if the $iif() was: $iif($1 != 5,$1,some default value) ? Your example of $1=some default value doesn't consider anything except null and wouldn't work for something else such as the 5 is in this example. Where would you put the 5 in your code?
How would you even read it? An IF/ELSEIF/ELSE or $IIF() can be read and makes sense when you read it... If X is true, then do this, else do that. Or, if X is true, use this value, else use that value. Your example basically says the variable equals one value equals another value. It really doesn't make much sense and I can't think of any language that uses anything like that syntax for comparing 2 items and setting a variable 2 different ways based on the result of that comparison.
