Sorry, you cannot use the "it's not a creature" for one spell and not use it for all spells. That defense of your interpretation doesn't work.
'Creature' is the same definition for each spell.
Mirror image has the potential to move the target from the valid (creature) to a (usually invalid) image, and only because it specifically says it can. It doesn't turn images into creatures, it just moves a validly targetted spell to a new target: an image.
There are a lot of different attacks in 5E that do not use an attack roll. Grapples, Fireballs. These are still attacks though and still use the Attack action. They are not attacks that use an attack roll.
If only there was some definition in the PHB....wait, there is! The question we are asking is, "Does
magic missile count as an attack?"
We have our definition: "If there’s ever any question whether something you’re doing counts as an attack..." Yes, that's the question we're asking! "...the rule is simple: if you’re making an attack roll, you’re making an attack"
And yes, Mirror Image typically only stops those attacks that have attack rolls because its mechanic uses AC to determine whether a hit occurs.
JC said, "The mirror image spell has no effect on magic missile, which doesn't involve an attack".
It's clear. To count as an attack in 5E, it must have an attack roll. Unless there is a written exception, such as grapples and shoves ("Using the Attack action, you can make a special melee attack to shove a creature, either to knock it prone or push it away from you. If you’re able to make multiple attacks with the Attack action, this attack replaces one of them"). Neither
fireball nor
magic missile have attack rolls or specific language saying that they are 'attacks', so they are not, RAW.
Unfortunately, just like Mirror Image trumps the "creature" language of Shocking Grasp, Magic Missile trumps the "AC" language of Mirror Image.
It has no language saying any such thing.
Magic Missile explicitly states that it hits its target. Explicitly. Mirror Image loses an image if the image is targeted (controlled by the Mirror Image mechanics) by an attack that "hit" it.
It does
not have language that says that it's an attack! MM works by changing the target of an attack. It has no effect on things that are not attacks. Spells do exactly what they say they do; it says it affects attacks, it doesn't mention anything about things that are not attacks, therefore it has no effect on things that are not attacks.
This is a special corner case. Hence the reason people are discussing it. Both interpretations are totally valid. Yes, you have your preference, but there is nothing that indicates that your preference is the only valid interpretation. People wouldn't be discussing it for so many pages if it were crystal clear.
First, people are discussing it either because they don't understand the rule or because they are still thinking about the way these spells worked in previous editions where MM definitely
could take out images, and MM was ideal for doing so. It just doesn't work that way in 5E, according to both the RAW and Sage Advice of the writer of those rules.
Second, although people can use whatever house rule they want, the actual rules for this situation cannot be legitimately interpreted either way. In this case one interpretation is correct and one is not.
Once again, "The mirror image spell has no effect on magic missile, which doesn't involve an attack", according to the final authority on RAW. Remember that this debate is about what the actual written rules are, not what anyone's house rule may be.
