I don't understand the idea that adventurers can't figure things out from, y'know, Experience. I've done that, and I don't have a game mechanic outright labeled "experience" tied to leveling up.
Who trains all of these trainers, anyway, if it's impossible to learn anything except by being trained by more powerful trainers? That also requires the campaign world to have a lot of high-level trainers that apparently don't actually use their powerful abilities--just teach them.
That's something that doesn't really sit right to me.
Hey SuperZero! Welcome to the board!
Please don't take a mocking tone with the members here. It will allow your points to be better received
As for figuring out things on your own, there's definitely a lot of RL examples to draw from (as you've already pointed out). As an electrical engineer, I'm sure I could have figured out everything I learned in college given enough time. But, that's the point of having a trainer, to save time.
A trainer can point out the 'tricks of the trade' and pitfalls without you having to learn them via trial and error.
So, sure, if a character says, "I want to be a wizard, but I don't want to use a trainer." I'd say sure, but it's going to take you a couple dozen months to master all of the abilities a wizard knows through trial and error. But, I certainly can't see someone just waking up some morning with all the knowledge of the wizard class.
Additionally, if players in my game have a concept which involves multi-classing, they can select an appropriate background to support the new class when they do make the switch. This represents the time they spent prior to becoming an adventurer training in the skills necessary for their new class.
Hopefully, this clarifies up my position.
Have a great day!