Well there are 2 sides to every coin, while I do agree in part that the other RC teams have been less of a competition for the AB's in recent years, I can't comment as much on the NH teams. If we look at South Africa, yes we did lose for the very first time ever on home soil to Ireland last year, but still we won the series, and the same this year when England came to our shores. When there were talks that we would lose the series ahead of the matches actually taking place.
But the issue is broader than this. How many times do the NH teams send a "B"-team for the June series to the SH, to experiment ahead of the NH season starting? Only to then get thumped, and then at the EOYT have a better team who then puts up a better performance against the battered SH teams who by then have been playing non-stop rugby since January???
What you are also forgetting is the change in personnel and other factors within each nation/union. I think SA again maybe has had the unique situtation where we fired a coach midway through his contract (something that last happened in 1997), and got an interim coach (for the first time ever) to build a team for the 2019RWC.
But the question is Who is NZ's stiffest competion now? Ireland? Maybe, but they only narrowly beated the Wallabies in June, and NZ comfortably smashed them now twice in a row. So who else? England? Same argument could be used with South Africa, who is yet to play the All Blacks this year.
That leaves us with France, Wales and Scotland. Out of those 3, Scotland might be the better of the 3 at the moment...
We are talking a country that lives and breaths rugby football competing with countries where rugby union is perhaps 4th or 5th in terms of popular sport interest among "natives", if any at all. I'd say darts is possibly bigger than Union in the UK at present. Many countries where there is no professional rugby presence at all. Rugby Union, as a world concern, gets very little appreciation. Certainly in my quarter of the world. In the USA, the world largest sporting economy, rugby gets effectively zero national interest. Competes with high paying sports like nfl, nhl, nba, mlb to name but just 4. What athlete, in their right mind, would chose to play rugby.
So this is what competes with a country where, upon birth, a rugby ball is place in your hands, and your are told to go and run. Male or female.
Top athletes in other countries do not get the same introduction as New Zealanders do, and if they are determined to be athletic, at an early age, then it's likely to be in a different sport than rugby, to which they are likely never exposed in the first place. While putting a potentially narrow view on the situation, the national sides of many countries are formed with what is essentially the best of the rest, in terms of athletic ability.
Ask yourself what are the top sports in any countries who have a chance of upsetting the ABs in a match. England, Argentina, Ireland, oz, France, etc. Rugby is likely well down the pecking order of sports interest for many of their people. New Zealand, it's the reverse. Everyone plays rugby, and those who do not excel, play something else.
My own rugby experience started out of chance, and against personal wish to play at a footballing "grammar" school in the uk. I was 11 before I even kicked a rugby ball. I already had 8 or 9 years to make upon my New Zealand counterpart. Plus I still had an urge to play football, so my time was split between two sports. Even while playing rugby at a decent level. Even then, All the rugby players I knew had other sports interests. Which might not be so unique in of itself.
In developing rugby nations, like the USA, I bet most rugby players don't start till their teens. And the best will likely face competition from much more lucrative pursuits than rugby union.