I think the thing that a lot of people are missing is that, while in an ideal world diversity is just something that should naturally happen, it isn't happening naturally a lot of the time. It remains unusual that top jobs are occupied by women, for example. The trends are positive but we aren't there yet.
There are presumably several reasons for this. Most simply, everything could be (almost) set up correctly now, with minimal or no discrimination when it comes to applications, and it's just a matter of allowing things to move along naturally. This is probably mostly the case, but there remain several companies where discrimination is a problem -- I'm not going to name names either, because this is probably one of those things that's hit-and-miss rather than actually systematic, but studies tend to suggest that the chance of, say, a black person getting a job increases if the CV application doesn't include a photograph compared to when it does, even if in every other detail the CVs are the same. There is such a thing as latent discrimination and it's unlikely ever to go away entirely. People just don't tend to trust people who looks or seem different.
It does seem that this trend is reducing, though, and the chances of getting a job are decoupling from your background now. On the other hand, you then hit the second and third reasons for a lack of diversity -- it can be that there actually isn't any discrimination when it comes to applying for a job, but the perception is the opposite, and people feel that they are likely to be discriminated against. This could be an historical hangover not helped by the fact that the profession in question doesn't "look" diverse enough. An example that has nothing to do with race and a lot to do with the old class system might be the Oxbridge application system. Historically there was a heavy public-school bias, so that there was basically no point in applying from a state school. This hasn't been true for decades now, but the old impression still lingers sadly, and it's reflected in the student make-up. Indeed, sometimes active efforts to encourage state-school students to apply are rebuffed by the schools themselves (this has happened). Oxford and Cambridge Universities are working to fight against this impression and it is having an impact, but they have to do that work to ensure that the applications are as diverse as possible -- that they can be ensured of having the widest range of candidates to choose from and ensure that high-quality candidates aren't being missed.
The third problem is that it is often true that the talent pool isn't diverse even before there's any job application. Black people tend to do less well at school, for example. Or fewer girls study physics and boys, and vice versa for biology. This may be linked to long-term perceptions of the "oh that's just a girls'/ boys' subject" variety. Again, it's important to combat and dispel these attitudes if they exist. It benefits everyone to ensure that as wide a range of people as possible are trained highly.
Despite all the above, the absolute worst thing to do is to enforce quotas. They remain horrible ideas. But if a workforce is not reflective of the diversity of the population then it does suggest that there is a systemic problem somewhere along the line that must be addressed.