An "affirmative action" policy combined with heavy investments in black communities would be better than a fixed quota. For example, when it is a close call between a Black and white player, you go with the Black player.
The fixed quota system has only hamstrung the national team.
The thing is that in the past the quotas were not this rigid and all the side had to show for it was one Makhaya Ntini in twenty years. Unless you actually force changes structurally with rules set in stone, grassroots changes rarely ever happen practically speaking. You need a catalyst to enact change ultimately.
And even with these rules, discrimination still occurs. Just a couple of years ago, there was an article that pointed out how black cricketers who were primarily top order batsmen had to play in the middle order, sometimes as low as seven or eight. Black all-rounders were often batting as low as ten or eleven and bowling only a couple of overs per day. This wasn't down to forced quota picks either, these players were legitimately not given proper chances at a higher level after proving to be competent previously. The current head coach led Titans to success but there were also accusations of racism in his own dressing room and he was part of the infamous 'clique' at the international level.