if anything it allows for more creative builds.
I'm not quite seeing as to how it encourages more creative builds since none of the effects changed. Only the method to obtaining them changed. Unless you mean that it more or less forces players to try out a new rune effect every level since they might as well anyway and there's really no reason not to.
One way that the old system would have promoted diversity is because no one's character progression would ever wind up the same way.
Assume two players do a no-twink ironman run using only the items that they find in game:
New system: Their skill progression will look identical, and their rune progression will be identical. Their gameplay experience will be exactly the same. Every single level up, they will always get the same skill learned or rune effect learned. The character's
entire destiny is set from level 1. There are no surprises. No hard choices that take into consideration each individual players' perceived value of different skills and runes. Everyone always has the same choice to make as everyone else. By the time you reach level 60 you will have everything available to you so who cares.
Old system: Their skill progression will be identical, but their rune progression will be random and totally different from each other. Their rune progression will progress only by means of what runestones the player can find. The only thing that is set in stone is their skill progression, but beyond that, their future is still unwritten. Players have to make real choices about which runes they choose to level up their skills since the runes themselves are a scarce resource.
Now, I acknowledge the real inventory problem, and there can be solutions to ameliorate that, but I'm putting that aside for now and just concentrating on the gameplay aspect.
The more I think about it, the more it feels like they dropped the ball with this new rune system.