It's not about complexity is about the flexibility of the spell casting system.You'll have to explain that for me.
"Infusions" are just enchant item spells. They have a long history in D&D. There is nothing particularly complex about the idea or which is particularly difficult for a spell to accomplish. There have been spells that turned mundane items into magical items for as long as I've been playing.
In fact, there is actually a very strong reason to have as spells things that would replicate the effects of magic items, because you are simplifying the system. You would not only no longer need to describe spells and magic items separately, as magic items could just reference the standard spell effect they produce, but you'd also simplify describing the crafting system for magic items.
Spell casting system is Is written in a way that if you allow them to create magic items using spell slots the amount of spell slots a caster has will break the system in half.
"Oh it's simple" until you attempt to write the mechanics down.