The nose concave gives two things as I understand it, an air pocket under the nose and a greater distribution of weight to the back of the board.
The concave does not replace rocker, rocker is a difficulty for nose riding as you increase rocker moving forward on the board begins to disengage the tail and fin/s, the tail will slide out or break free causing pearling.
So flatter rocker maintains the wave lock on the tail and provides a more stable platform to stand on when forward.
Rocker also shortens the length of board in the water which means the board has less glide and is harder to paddle as the effective length is less.
Turn requires a different technique and positioning on the board and does get the same shortness of arc of a short board, you can increase turning ability by increasing tail rocker but again you have the problem of lifting the tail and disengaging the fin/s when you go forward.
Walking the board by cross step is a way of moving forward that keeps the tail engaged and maintaining the stability as much as possible.
You can nose ride without the concave, the rails may need tweaking but the older school noseriders had egg rails or 50/50 rails but you had to have your longboard turn skills up.
Watch a lot of old surfmovies watch how the guys turn and set up noserides in various ways.
In particular watch Phil Edwards, Dewie Webber and Miki Dora then realise that even then longboards could be turn well, modern longboards are just that more advanced now, just don't try to turn them with shortboard techniques
I've taken up troll hunting just for fun, instead of a rifle I'll just use a pun! 冲浪爷爷