This is all about sales opportunities. And the way to sell this is access to 60+ new resorts, flexibility in usage, and creating fear that what you have now will go away.
But all that has nothing to do with taking inventory from VSN into the DC. They can sell the DC to existing MVC and VSN owners even if they don't have a single Vistana week in the DC inventory. The point is that, once this is launched, they can legitimately advertise that you can find Vistana weeks in the DC exchange (which would be true - there will be VSN weeks there if owners elect DC points), they can say that what you have now will/can go away (which is true if most VSN owners went the DC exchange route) and they can say a lot of other things that would technically be true if certain conditions materialized. But there is no reason to grandfather weeks unnecessarily (from their perspective) just to get inventory. Likewise, there is no need to artificially move any VSN inventory into the DC because if you don't find VSN weeks in there you have no idea why... Did they get picked up by another owner? Did that inventory never exist? Maybe someone else had those dates on a waitlist? Lack of inventory is actually another selling opportunity - "buy more points and then you can book it because you will have a higher Elite status!"
VSN inventory may get impacted over time but I would attribute that to owner behavior rather than MVC playing tricks on owners. They don't have much to gain from that.