CapnZapp
Legend
The harsh truth is that +5 is equally powerful and desirable at every level. While the cost of a first level spell slot only diminishes as you level up.Shield is ... problematic. The problem is that a d20 has so much variation compared to the delta of to hit vs. AC under bounded accuracy that you need a high bonus to overcome the die and make it worthwhile in the first place. But without needing to be upcast it scales very well, and it's virtually a get-out-of-jail-free once the resource cost of a 1st level spell slot diminishes. Also as a 1st level spell it's very easy to pick it up via cherry-picking.
By this logic Shield should require a higher spell slot in order to provide defense for a higher level character. Let me illustrate:
An invisible barrier of magical force appears and protects you. Until the start of your next turn, you have a bonus to AC equal to 7 minus your proficiency bonus (maximum 5), including against the triggering attack, and you take no damage from magic missile.
At higher levels. When you cast this spell using a spell slot of 2nd level or higher, the AC bonus is increased by one (maximum is still 5) for each slot level above 1st.
At higher levels. When you cast this spell using a spell slot of 2nd level or higher, the AC bonus is increased by one (maximum is still 5) for each slot level above 1st.