As a resident of Nevada, I can tell ya that legalization doesn't immediately lead to a wonderful, exploitation-free situation. Driving by and seeing what the brothels here look like is depressing, more often than not. And, quite frankly as someone who has had many close friends who either are or had been sex workers, some of this strikes a personal tone for me.
I can't speak for Rick, but I will say that as sad as the circumstances around even legalized prostitution can be, the point Rick and many advocates of liberalization on the issue are going for is that further liberalization is what is needed, rather than interpreting the existing flaws of RLDs as evidence it's all hopeless.
Even when legalized, the fact of the matter is sex work is often stigmatized. That stigmatization relegates it to the fringes of society, and that makes it very susceptible to exploitation and the involvement of ill-meaning types (organized crime, abusive pimps, human traffickers, etc). Understand that sex workers - even when engaged in entirely legal work, regulated, taxed, licensed, and so on - may be unable to accept payment through regulated banking systems; the often cash-only nature of the business leaves it very prone to abuse. Sex workers may be denied concurrent, non sex work if their other jobs are discovered. They can be denied educational opportunities, custody of their children, the benefit of the doubt in divorce proceedings, social services, medical care, so on, so forth. And again, all this for doing work that may even be entirely legal where they live and work. Using an even more recent example, in the United States provisions were put in to various COVID relief bills to expressly deny sex workers access to CARES act relief, they can't get unemployment assistance, and so on. (FYI I'm including not just prostitutes when I say "sex workers" but performers in videos, dancers, and so on).
So again, it's that stigma that - despite legalization - can make for a situation ripe for exploitation. So I, for one, am glad Rick brings attention to the issue because I think the only way we get to where the exploitation isn't occurring is even greater liberalization and acceptance to the point it not only carries no real stigma but isn't even a curiosity.
Same goes for legalization of certain drugs, or alternate approaches to dealing with the societal problems that come with them. I don't think any proponents of weed legalization think we just get to flip a switch and all is well, for instance. Rather, we understand that decriminalization can have sweeping effects on society, and especially in marginalized communities who are often disproportionately targeted in drug enforcement policing and prosecutions; something like a needle exchange program can greatly limit the spread of bloodborne pathogens in using populations and benefit the community as a whole, and threatening to round up and lock up the folks showing up to exchange their needles is counterproductive; while far from perfect, certain protections and expanded access to healthcare exists for a legalized sex worker that simply would not exist for someone operating in a criminalized profession.
(continuing in another post cuz I'm running out of space...)