It is not free, especially if it takes a 10 minutes to cast an action spell, or an hour to cast a ritual. "You guys wait around for 10 minutes while i cast knock on that chest."Except the core rules give you that extra utility outside of combat for free, and don't require a trade-off.
I'm coming at this from the premise that it's obvious that casters are overall more useful and powerful than martials in core 5e. If you think they're actually balanced, than obviously this sort of project seems flawed. From my perspective, you could add 10 minutes to the casting time of every spell in the game and wizard would still be a useful class.
Your premise is wrong IMO. Rogues are the most useful class in 5E and Rangers are a distant second (that includes their spells but I am not considering them casters for this discussion). These are also two of the weakest characters.