I think it's a case of recognising that while decent, we're a way off the best teams in the world. Being good in the Six Nations counts for very little now France are pants. The main problems being:
Lack of depth at 15 -- Brown is 30 now and is already a slow international player. By the Japan World Cup he will be 34 and into Cueto-levels of slowness. Only Goode is a full-time fullback at club and he's repeatedly shown he lacks the athleticism to be an international 15.
Wingers -- only Watson and Nowell have consistently shown international class. We've seen flashes of it from Yarde, May, and Rokoduguni, but otherwise we're one injury away from putting in a rookie/liability. Nowell's lack of pace combines badly with Brown's.
Again, two issues here:
1. The last time a team won the Grand Slam and went to Australia (Wales 2012) they got whitewashed by a weaker Wallaby side. Beating the NH does nothing more than get 4th in the world rankings. That's more likely to become 5th, thanks to Argentina, than 3rd, 2nd, or 1st.
2. Just because we have a young, inexperienced team doesn't guarantee progress. Lancaster's team peaked mid-2014. After that Twelvetrees, Burrell, Farrell, Yarde, Cole, Lawes, Care, and goodness knows how many others all got steadily worse. The difference between the play in NZ in 2014 and the RWC is staggering at times.
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Long story short, we're a long way off being truly good and having proper depth (i.e. several, proven options) across the squad. If we rest on laurels after one Grand Slam we'll become Wales 2.0.