1) you are not one of those people whose entire or the majority of their 4Ed material is not DDI only and/or
2) WotC- and such successors in interest as there may be in the next 30+ years- actually do keep that 4Ed DDI material online all that time.
I don't doubt that there are folks who primarily accessed D&D 4e materials through DDI.
However, if they went into it with the expectation that it would somehow last forever, or even particularly long, I am not sure they have anyone other than themselves to blame for their situation. That editions change was pretty clear. That the Internet is not static that services come and go frequently, is also pretty clear.