I think definitely if the proper diagnosis is made early, or even at all, it increases the chances of a more "normal" life for people on the spectrum in the long run.
I look at the leaps and bounds my son has made in the past 3 1/2 years, and it amazes me. But if he had never gotten that diagnosis and the help he needed to help him progress, who knows where he'd be even now at age 5? I picture him never learning to speak. I picture him having constant fear and anger. I picture him falling further and further into a world in which no one but himself exists. I picture us not being able to take care of him and unfortunately having to put him in a home. And that's what happened years ago to many an autistic patient and family.
Luckily, we've had assistance in one shape or another since he was 20 months old (although his "official" autistic diagnosis didn't come through until he was 4, but we knew by the time he was 3 and in pre-school - in the beginning, it was simply a severe speech delay diagnosis, but that was good enough) and that has helped him to learn, grow and move forward in life. It was baby steps at first, but then, things began to snowball.
He is in Kindergarten now, reading and doing math at about a 2nd grade level. He is in a reading club!! His social skills, while maybe a bit awkward, are better than I've ever seen. His speech grows with each day and he's speaking at about a typical level for his age (if not always appropriately) even though he didn't say his first word until he was nearly 2 1/2. There's talk of total mainstream at his school, with autistic support along with his obvious continuing speech and occupational (and maybe behavioral!!) therapies... and we're totally OK with that. He's spent a day with the mainstream kids already and did quite well with only one "shutdown". He already spends part of the day with the mainstream kids for reading/language arts and PE.
All of this may be by chance... who knows, he could have done all of this on his own diagnosis or no, help or no. All I know is how he was before we got him the assistance he needed and how he was after. He still has his moments, trust me. We deal with them, as any parent does. But yes, in my opinion diagnosis is key.
Does this make it all go away after they grow up and move on? No, I'm sure it doesn't. I'm sure even as an adult my son will clamp his hands over his ears when he hears certain noises... or at the very least, if he has learned some "control," he will cringe until the noise passes because I'm sure he'll still be sensitive to noises even as an adult. I'm sure he'll still be eating the same old familiar foods even as an adult, I bet that gustatory sensitivity will always be there as well. And I bet I'll have to call him to remind him to eat, because he seems to have no hunger cues and never has. I doubt that'll change much. I'm sure he'll spend some of his day bouncing/jumping, running or spinning to get out that vestibular seeking need and endless energy he has. I'm sure he'll spend some of his day admiring and playing with his collection of cars, complete with the stimming "vroom" sound that goes with it, if he's alone that is, to take care of that vocal/auditory stim.
But with all that said, I bet you he's going to be one hell of a guy regardless, all thanks to all of the doctors, teachers, assistants, therapists, everyone else who's helped us along the way and that one piece of paper that gives him his diagnosis and opened him up to all of this help he's gotten.