Criminalization is not a rational policy, it is based on emotion and money. However, I think it is safe to say that legalizing prostitution would reduce the income of certain prostitutes. The sex workers I have spoken to seems to want the benefits of legalization but none of the disadvantages (like possible market competition). Now prostitution is often controlled by organizations that decides who will work where, which is very beneficial for certain sex workers that work outside the syndicates in high end-markets.
Imagine for example you are a weed farmer and instead of selling it for 10 EUR per gram you have to sell it for 0.1 EUR per gram or less. Not as fun.
Although it would end up balancing out. Those who aren't happy with the income would leave the industry increasing the low income because theres less supply until those who stay are happy/willing to accept with the income they are earning and at the same time their lives won't be at risk every day.