Today, I got in 600 Fuji Velvia 4X5, some for me. Maybe if Fuji finds out there is no real demand to replace 50 with the new 100, they will keep it on the market. Remember when Kodak decided to replace Plus X and Tri X with Tmax films. 20 years or more later Plus X and Tri X are still on the market. Stock up on 50 just in case.
Go to the Fuji web site and tell them you want Velvia 50 to remain. Those who want 100, that is fine, you already have it. As someone said in this forum very recently, if it is not broke, don't fix it.