First of all, let me say... I was lucky enough to see this in the theatre in 1977 and it remains, in my opinion, to be the greatest film ever made.
Secondly, there is a lot of talk (even from Spielberg himself) about how wrong it was for Roy Neary to abandon his family at the end. Spielberg has said he probably would have rethought the ending after having had kids of his own. WRONG!!! It's Roy's very decision to leave all that is earthly behind in order to take the trip of a billion lifetimes that makes the film so iconic.
The Special edition was unnecessary. What we don't see in in CE3K is what makes the film so incredibly powerful.
Roy was a seeker. Once he'd experienced his close encounter his ordinary suburban life became less important to him and his search for answers became an obsession and his life goal. People including Spielberg himself now berate his decision to leave on a voyage of discovery to the stars but he's no different to seekers of old who also had families, like the explorers and conquistador's of old, some who would be away from home for years on end. Some folk are way too hard on Roy, he was given an amazing opportunity and chose it over a life of mediocrity.
Obviously he was a seeker but that doesn´t make him any less selfish. The guy was a self-centred scumbag. Whether his life was mediocre or not, isn´t relevant. A lot of people live mediocre lives but it doesn´t mean they should feel entitled to abandon their wife and kids.
Roy's experience is emotionally a religious or spiritual one: he's seen the light & he must follow it. His life has been utterly transformed. He MUST be true to that, or his remaining life will have no meaning. So much of human art, culture, thought is the result of people having had such experiences & then pursuing their call, no matter what the cost. It's something that washes away one's previous life. How could Roy have gone back to what he had been living, knowing for the rest of his life that he'd given up a transcendent experience that would have remade him? It's the Hero's Quest: a person is called from ordinary life to go out into the unknown, face unimaginable mysteries, survive them, and finally return with a great boon for all of humanity.
And it's not like Ronnie gave him much of an option. He tries to patch things up with her on the phone in an earlier scene, but the call goes further and further south until she hangs up on him. At that point, I think he realizes that he's lost Ronnie, the kids and his job, so there's basically nothing left for him on Earth. From then on, there's no going back. As you said, he had to stay the course, or his life would have no meaning. As he said to Ronnie when he was shovelling dirt into his kitchen window: "If I don't do this, that's when I'm gonna need a doctor!"
Agreed! And I can understand Ronnie's viewpoint, too. Roy's experience has changed the course of his life so utterly & irrevocably that their previous life together can only dissolve. There's no going back for either of them.