I can't speak to if/how 5E was nerfed in relation to 3E or before. I just know that one of my huge problems with the game pretty much since its creation was the fact that Wizards (and clerics, etc,,,) had spell slots at all. I get that it was made into a resource that was designed to be depleted and refreshed. It still bothers me that a fighter can swing their sword (effectively) all day long whereas the wizard is limited to a very small handful of casts per day. In that, I like that Cantrips can be used to some effect. It gives those casters something to do the other 98% of encounters.
But consider that D&D evolved from tabletop wargaming, I always felt that the spell system was built on that concept and never evolved, as it should have. Maybe it was time to show some development (beyond what we have already seen). Considering it is now a CRPG, something had to give.
As far as Cantrips being exclusively harmless magics used solely for the training of novice casters, I don't necessarily see it that way. Maybe there's support for that line of thinking, I don't know. But with everything in magic, there is always experience, knowledge and the ability to improve something. It seems to me that should go for cantrips in the same way as any other magic. Cap it, sure. But, within guidelines, I see no reason why even basic magics can't be improved with experience.
