This competition was created with the aim of ending friendly games. Therefore, it is normal for many players to take these games less seriously. As the competition is still very recent, it still doesn't generate much motivation. But with three or four more years of competition, I believe that mentality will change a bit.
I don't think the whole purpose is to completely eliminate friendly games, but to instead provide a more organized structure to all the friendlies that can happen in Europe, where there are many, many great teams.
Friendly matches on top of the Nations League will continue to happen, just not as frequently as before. In a way, this makes friendly matches a bit more interesting, because there are less of them. The Nations League is basically a lower tier tournament compared to the World Cup for example, but it's nice to have so many different teams playing with each other all the time.