But no one really has to upgrade do they? Nothing needs to be done to the current bikes people are riding. And even 25% of new bikes offer discs. And I'm dead sure they will still be making good quality caliper brakes, rims and hubs 10 years from now. They even make cantilever brakes these days, and we all know how much those old fashioned things suck... There's even a long arm caliper on the market today.
Discs only really concern people who want to buy their next bike with discs (such as me)
Not even people buying a new bike need to be afraid of the industry cramming discs down anyone's throat because of the 25% portion of disc bikes. There is literally no risk in this.