You make a lot of good points and fair to see it as a huge move when looking at it like that. If we're talking as a development platform, there's very few destinations greater than Sporting. They are recognised as a development factory, globally.
However, they don't play in any of the Big 5, and have never won a European Cup, which for me is a criteria for a huge move (albeit Sporting is a stronger move than Nottingham Forest, Hamburg or Marseille).
They're not bigger than Dortmund or Villa.
They're certainly not bigger than the powerhouses of European football (Liverpool, Real Madrid, Barcelona, Bayern Munich, ManU, AC, Inter....the list goes on).
Was my point.