How much longer do you believe adaptors will survive? I own a Letus and am very happy with it. Is it possible adaptors may survive after all when people realize it gives them a vintage look?
Adapters will survive as long as people want to use them. That may sound like a ridiculous response - but that's what it comes down to.
Look at the DVX100, it's still not only being used - but still being sold (and it was released in 2002).
Many people will always jump to the latest and greatest - but there will always be those that continue to create great work with the tools they already own.
So - while I do doubt they'll be sold for a long time to come...I think they'll be used as long as people can find creative ways to use them.
Point is - your gear is always relevant if you're creating engaging work with it.