For games where the number of challenges approximate the 6-8 encounters*, Shield is fine for regular ~AC 12** (15 with Mage Armor, an additional expenditure) wizards boosting their ACs to ~17/20 for a round (there are just too many cases where the boost isn't enough that you can't rely upon it), as well as for Eldritch Knights who boost their ~17-21 AC to 22-26 (EKs just never get all that many spell slots, if they spend them all on situational AC boosts it is perfectly reasonable, and if they also have a physical shield they need war caster as well). Also some other straightforward (right out of the book) ways that people get shield (ex: hexblade warlock, where a spell slot per SR becomes a huge expenditure).
*and the whole policing the 5-minute workday is a very real issue, but one I consider larger than just this one spell.
**all ACs with the parenthetical "plus potentially a few more if you have rings or protection or the like'
Where things break down is 1) the aforementioned workday issues, and 2) all the other situations where PCs with relatively large spellcasting pools also have ACs in the 17+ or so range (Cleric or Artificer1/Wizard X-1, various races with armor proficiency or armor-replacement AC, various ways to borrow spells from other classes, etc.). That's where things kinda go off the rails. It also means that various ways to pick up spells can't or shouldn't be included (or at least it will have interesting consequences), and that potentially has constraining issues on the game design -- examples might be how there is no equivalent to Fey Touched/Shadow Touched feats, but for abjuration*; or how one D&D is giving Bards (including presumably the well-armored valor bards) the arcane caster list, but not abjuration (losing them dispels).
*the Dragonlance UA had some, but who knows where that lands
All in all, Shield doesn't strike me as one of the worst offenders in the poorly-thought-through spells, but I would rather the implementation have been different. Mostly because of the constraint in other design aspect. I think workday and MC dips are something that the game overall needs to address (if nothing else move Multiclassing to the DMG and put warning labels around).