"What do you think the view of our Afghan campaign will be in say 20 years time?"
That's impossible to predict. It will depend on events in the interim, and in 20 years an awful lot can happen. twenty years ago Bill Clinton had just been elected, the Channel tunnel was opened, apartheid came to an end in South Africa, and the first text message was sent.
The next twenty years will no doubt see other big world events, and the current Afghan situation will be viewed from a totally different perspective.