Agreed.
People who say "so what if Vega is 10% slower than the 1080ti? It can be cheaper!" are missing the point. The problem is that, according to the current roadmap, a refreshed Vega is all AMD has in 2018 as well. Thus, Vega MUST be able to at least somewhat compete with Volta, which means it must be a good deal faster than GP102.
Sadly, AMD is not inspiring confidence in me with the way they act when it comes to Vega, and the late launch. I think of how they showed Ryzen exchanging blows with the 6900k months before launch, and then I look at Vega with shoddy CrossFire benchmarks and vague statements about its performance. As you say, I think 2018 may be a disastrous year for Radeon and that we won't see it bounce back to finally being competitive across the whole product stack until Navi+1 or Navi+2...