I agree with you that some cantrips - mold earth especially - would be very useful. But I don't think that a cantrip could be cast every round, minute after minute, hour after hour - the caster would just get exhausted. Sure that's not in the rules, but the rules don't cover every situation, and we can use logic/common sense. The same way a person can't swing a sword hours on end, a caster shouldn't be able to cast cantrips at will forever.Consider what a 1st level wizard can do with cantrips and rituals. In an 8 hour day they can do the work of dozens, if not hundreds of people. Spend a little time actually planning it out for what could be done on a farm and it is really interesting. Is it cheaper to have 30 people woring on your farm or 1 1st level wizard.
that being said - even if the caster is using a cantrip every 5 min, that is still extremely useful.