Arguably, mandatory algebra is a good idea because it may be difficult to identify career-algebra-users in advance, and they may have a disproportionately large positive impact on society relative to their numbers.

I also seem to recall something about taking a high school algebra class improving one's thinking skills (even if you don't do well, apparently), but I haven't been able to find the cite for that.

Here in America we like to maintain the illusion that everyone's cognitive ability is equal and with the right education the playing field will be level. This is indeed false, but I do wonder if it's a useful illusion. I've noticed that reading research relieving me of this illusion has caused me to become less empathetic and more cynical. Another reason to promote this illusion: research on fixed vs growth mindsets.