Aside from the obvious out-of-universe answer, let's look at it in-universe: Wouldn't it have been more wiser of the Pokemon Professors to give out starter Pokemon and a license to catch more to those who know just about all of the basics from the get-go and are capable of surviving out in the world independently while raising creatures who could very well kill you in a wild encounter? I mean, Ash was lucky to come across Misty and Brock who helped them but suppose he never did. He wouldn't have made it out there all on his own. If he was more smart, resourceful and independent (like Red from PokeSpe), then I could totally get him becoming a Pokemon Trainer since Oak (we don't know about Sammy from the fourth movie, mind you) would've seen him for the role. Plus, the "ten year old" thing could just be written off as people in the Pokemon world maturing faster than in our world. Plus, there are Pokemon Trainer Schools and yet Ash seems to have never gone to one. They should be a requirement, not an option.
Same goes for May and Dawn who were even more clueless. Dawn didn't even know how work a Poke Ball until Ash told her to make it bigger. Imagine if Ash and he never met.
So what are your thoughts? Please refrain from answering with things like "it's a kids show so they need this to make relatable characters" to try and brush it all under the rug and leave it at that. At least provide more of an answer and tell us your opinions on this from an in-universe perspective as well.
If Ash had never met any of the people he did on the start of his journey, he'd have given up like some of the others. Not everyone can train Pokemon, and if Ash hadn't met Misty, Flint, or Brock, then he wouldn't have been able to get anywhere as a trainer.
May would have been able to make it simply because her father is a gym leader. If Ash hadn't trained her or allowed her to observe battles, Norman would've. May knew how to battle going in, but she was thrust into the situation quite quickly and had to adapt to actually traveling and having intents and purposes for it. She became a fast learner, and although her stage fright at first was a big deal, she later made the best of it and started winning contests like there's no tomorrow. If Ash hadn't been there, her father, wanting her to go on a journey, would have helped. He wanted that growth and was determined to get it.
Dawn probably never would have made it. Her mother was a decent trainer and could have helped, but not to the same extent. I mean, if Johanna hadn't even shown her how to work a Poke Ball, how effective would she be? Dawn had no idea what she was doing the entire time until she got some help. She hadn't had it yet.
One of the biggest problems with the anime is that it's so concerned with battling and, in some Gens, contests, that it doesn't give itself any room for character development. We only see subtle growth (although there was plenty with May), instead of journeys with intent and substantial character growth through an arc; the biggest problem due to that is the re-use of characters in each generation, as opposed to each generation having its own standalone story. They want those relatable characters, as you said, and that is a problem. We don't get to see the fierce determination and the backstory, or the WHY. We can only theorize for the most part. I don't recall why Johanna wanted Dawn to go on a journey, maybe because she had and contests etc. It isn't as much like Norman wanting May to discover herself.
But like with real life, what if ___ didn't happen? What if Ash had been on time to Oak's lab and got Squirtle? What if Brock had stayed at his gym? What if Team Rocket never attacked the Pokemon Center? What if Ash never beat Lt. Surge? What if Pikachu never came back after Ash wanted him to go into the wild? All of these questions... it's hard to answer them.