> When you have an expression P which names a mutable place, and you execute P := X

This isn't the case, though, is it? A normal member access (or indexer) expression may point to a mutable location (field, property). However, with conditional access expressions you get either a member access _or nothing_. And that nothing is not a mutable place.

When you use any of the conditional operators, you split the following code into two paths, and dropping the assignment (since there's nothing to assign to) seems pretty consistent to me, since you'd also drop an invocation, or a property evaluation in similar expressions.

If you require P to point to something that actually exists because you want the assignment to succeed, then write code to ensure that P exists because the assignment has no way of knowing what the intention was on the left side.