I suspect there might be a few more cases like this, but there will be a limit to the trend. One of the main ways free dating sites make money is by serving ads; ads for paid dating sites. So, the situation is a little bit like a miniature version of what occurs in capitalist societies. That is, if they get too generous with their welfare, and people prefer the dole to working, the economy suffers hugely. The government has to tighten the purse strings eventually and reward business instead.
So, unless things change radically and the free sites find completely new and very reliable ways of making money from their traffic, paid dating will always be around.
UPDATE: Commentor Fred below has real insights into the marketing of big dating sites. It seems that content ads not working now as a means of generating profit for free sites. The landscape is clearly changing. So, it will be interesting to see what happens.