I think your right in that as long as you get the frame size in the right ball park, the rest can be adapted to suit what comfortable for you.
That being said, I think its more important that you do adapt the positions of contact points so that you are properly comfortable on the bike. On a roadie, you’re more likely to stay in the same position for long periods grinding out the miles … so if you’re not comfortable / in the reight position to suit you, then it could cause problems the longer the ride goes on.
In the last couple of years, I’ve have to raise the stem/bars up a little bit to ease off straining the back from being bent too far over. I’m only talking about raising the stem about 1cm tops and tiling the bar upwards a little bit , but it made a big difference in the comfort of the bike.
So yes, fit is important, but to me, its fit of the contact points more that fit of the frame.