Certainly, mages should use tactics and careful planning as much as possible, and may not *need* the best armour. With that said, being able to make more mistakes and still not die is a definite advantage, and there is a huge difference between how many mistakes you can afford to make with 13 + dex mod as compared to 17 + dex mod(max of 2) or even higher AC with magical armour/shield. I certainly can't say for certain that medium armour plus a shield would be the only option a wizard or sorcerer should ever take, but I do consider it an extremely strong option if it stays.

Spells like expeditious retreat or invisibility don't even enter the equation for me. If I'm a wizard (or other spellcaster), concentration is too precious to be exclusively devoted to protecting myself in every fight. There will absolutely be fights where it is worth it of course, but it can't be my plan A otherwise there isn't much point in being a spellcaster outside of the handful of levels where fireball is king. If my concentration has to be used on expeditious retreat, that might be tolerable at level 1 when I don't have as many strong options to concentrate on, but at level 3+ it means I'm not casting web, hypnotic pattern, fear, etc. Considering my ability to cast spells like that is precisely the reason I might be a priority target, it makes no sense to give up my ability to become a major threat to use a spell that will protect myself that only matters if I am a major threat in the first place.

And I also agree that combat does not need to be fair all the time. The people attacking the party should make reasonable decisions, and their objective is not to give a fair fight. Naturally, that will look different depending on what is attacking you (a giant spider probably won't know what a wizard is, but a hobgoblin probably will) and choosing to target anyone who has less ability to protect themselves while posing a major threat should expect to be a priority target.