From personal experience - no. My father did just that many years ago. I’ll never forget the FBI giving me a two hour interview (and yes, they interview family that they do not consider suspect too). And while we were searching Iowa he gave up and called home from California. Point being, it’s totally easy to just leave without a trace. It’s not so easy to stay disappeared for long - and personally I think Mollie and a person unknown are having that very discussion hundreds of miles away right now.
First off, it's different in 2018. The technology (especially social media/location analysis) and video equipment available for a search today is vastly better than it was even 5-10 years ago. It is extremely difficult to vanish without a trace, because it takes money, experience, and knowledge that most (especially 20 year olds) don't have. It would take a very special person to pull it off, and would take months of planning. Most runaways are found within a few days. Mostly because it is so difficult to get away without leaving any sort of digital footprint, and because it is so difficult to break off all contact with everyone you know.
And second, I've said this before, if she had just run away voluntarily she would know by now that there was a major investigation into her disappearance. If she wanted to stay gone, then all she would have to do is get in contact with the FBI/DCI and tell them that she didn't want to be found by anyone. They'd verify that she was alive and well, and that would be the end of the investigation. And yet here we are with 30-40 FBI/DCI personnel on her case.
A voluntary disappearance just doesn't make sense.