I think the issue with Shield is it is an iconic spell that is very popular.
It is pretty difficult to dial back popular options while at the same time trying to increase the popularity of the game. There are numerous options that would make Shield a more balanced spell, for example:
1. Works on one attack per spell slot level (so 1st level slot the AC bonus is in effect against the trigger attack only, cast at 5th level it works on the trigger attack and the next 4 attacks before your turn ..... so if you are soloing a Malarith you would cast it at 8th level
2. Keep as is, but can't cast spells on your next turn after it is cast.
3. Make it flat 20AC, +1 for every slot above 1st.
Any of these make it far less prone to abuse, however at the end of the day while Shield is complained about by many DMs it is also THE MOST POPULAR spell in the game and DMs who want to actually find players to play with don't usually ban it even if they don't like it. Players who admit it is OP also routinely take it.