Even the Mk8 will be evolution rather than revolution.
It will look cosmetically different but in terms of driving experience, probably won't actually be much better, if at all.
It remains to be seen if they will even make a true pure petrol performance model, likely as not all of the engines will be 48v mild hybrid with the associated weight gain.
However, desirability on resale and the resultant price and success of sale is always better when you are closer to the current model and not a Mk4.
Besides, as I said, I'd take a new car over a secondhand one every time - I don't want someone else's farts in my upholstery