Valid concern. It's really hard to predict the outcome since the release of a legacy server will without a doubt attract a lot of retail players given it is released at a point where most people have done the existing content in retail. Let's say it was released today. Most people are having a break untill Legion, or are casually playing. If the legacy version would require a subscription (99% sure it will), the current playerbase combined would skyrocket for a few months, and then drop down once nostalgia wears off. I'm basing this off this graph:
http://www.misplaceditems.com/rs_tools/graph/
It shows the playercount when Old School Runescape was released. Notice the decline of players as the time goes. This will without a doubt happen to legacy servers as well. The question is if the the players who chose legacy over retail will return to retail when the next expansion/patch is released.