The thing is? Shield would be perfectly fine if casters didn't have ready access to Constitution save proficiency or better armor proficiencies.

Theoretically, arcane casters have to split their stats between their casting stat, Constitution (for HP and concentration), and Dexterity (for AC and initiative). You're probably going to have a Dexterity of 12~14 for most of your career, so Mage Armor is going to give you a base AC of 14-15. Slapping Shield on top of that gets you to AC 19-20, which makes you roughly on par with a martial character at the cost of a low-level spell slot and your reaction. It's fairly obvious to me that this was what they thinking of when balancing the classes - you cast Shield to prevent losing concentration on your big spell, because your AC and Con save are generally poor.

The thing is, even when you factor in the fact that casters generally hit a point where they don't have any good uses for casting low-level spells other than casting Shield and other defensive spells, that's still fine, because it mirrors how people with "real" armor are going to generally improve their AC, due to finally affording stuff like full plate, maxing out their Dexterity, or just plain finding magical armor. By the time that Mr. Wizard effectively has access to their Shielded AC all day long, everyone else should have AC that's at about that same level.

The problem is that they made it way too easy to get access to actual armor, which lets Shield break bounded accuracy like a twig. They made it too easy to grab proficiency in Constitution saves, which means that casters aren't forced to split their stat increases between Dexterity and Constitution to be "comfortable" in combat. While being able to cast spells in any armor you're proficient in is certainly a problem, the real issue is that spells like Shield were written without taking that into account.



An even bigger problem, in my mind, is that spellcasters are designed to cater to multiple incompatible playstyles. They have to have tons of spell slots for the people who want to participate in fights by throwing around multiple Fireballs, while at the same time they're designed so that you can get away with only using one or two spell slots per fight so that you can use them for utility. There's also a really bad trend where spellcasters get spells that boil down to plot powers - there's no good reason why things like Clone or Wish need to be player-accessible.

Honestly, the Warlock is closer to how spellcasters should have been designed if the whole "save my spells for utility and the big fight, normally contribute with cantrips" playstyle was deliberate. Because that's the playstyle they're are more-or-less designed around - that's why they get those Invocations that give access to additional spells known that they can only use once per long rest, and why they get bonuses to Eldritch Blast that make it as strong as it is.