Whether a World Cup win is realistic is besides the point. The question is whether supporters of Scotland, Wales, France, Ireland (and England) would prefer to win a first World Cup title (or a second World Cup title) or another Grand Slam. I think the answer is clearly the former.
However you have just demonstrated the lack of ambition so evidence amongst NH rugby (particularly Wales, Scotland and Ireland), basically, 'hey, we aren't going to ever win a world cup, so let's focus on winning the 6 Nations'. This is also evident in the structural lack of ambition in the very concept of the Lions - 'hey, we'll never bear the SH team on the own home soil so perhaps if we put together 4 teams we might have a chance'.
Rugby is a minority sport pretty well everywhere (arguably not in NZ and Wales, but they have tiny populations) - there is no reason why any of the 6 nations sides (with the exception of Italy) shouldn't believe they can win the world cup. Why are SH sides so inherently better that the likes of Scotland concede that they can't win before a pass is thrown.
You seem to want to say Scotland fans would rather have a World Cup win than a Grandd Slam but if they don't they are lacking in ambition. How is that tasty cake yoy. are having and eating?
And you seem to have little understanding of sport. To take an example, Greenock Morton after 12 games this season had 8 pounts and were 6 points adrift at the bottom of the league. At that point I had no ambitions that we would win the Champions League, nor even hain promotion, the aim was not to be relegated. After 22 games we now have 32 points and are in the last playoff position. The automatic promotion position has 45 points currently. At sometime in our run of 7 wins and 3 draws, I decided we were safe from relegation and started hoping for a play off spot. Winning the league though is still not a worthwhile ambition. If we take 24 points in the next 10 games, let's see. Winning the Champions League or even the SPL still not ambitions.
Now as to Scotland in rugby union, we haven't the Six Nations ever, never mind a Grand Slam, and it's a yearly competition. In order to consider the World Cup, we should be winning the 6 Nations regularly, with the occasional Slam. Till then talkung about winning the World Cup is unrealistic.
You see as the Morton tale of the bank goes, these things come in stages, and in addition just to help you out the World Cup only happens every four years so.as a fan, I'll take that Slam as possible though a tough ask, before any talk of the World Cup. Happy to have helped you begin to understand sport