Its not about loyalty though it is about all top top players should want to test themselves at the top level in their prime and he didnt.
Had he left and gone to Man Utd etc no Celtic fans would accused him of lack of loyalty etc. It was the easy thing to do to stay and be the hero in Scotland where can bang in 30/40/50 goals a season.
Nothing to do with Nolito or Soriano he was miles better than they are but Larsson in his prime did not have any 'world class' seasons as he had it too easy in Scotland.
Great, great player mind you.
Totti had offers from Real Madrid in early 2000's when Totti was one of top 5 players in the world and the best Italian player.
Real offered "gold" for him, and from 90% of people outside Italy, he should have accepted it, go to Real, "test" himself in the biggest club in the world in that era (CLs in 1998, 2000 and 2002').
Also, Milan was also offering 50+ Millions for him in those days.
That is easily 70-80-100 Millions today, like Neymar's or Suarez's price.
Totti said no.
He always said that he will never leave Roma, no matter how good/bad Roma will be.
He would probably had more successful career in Milan or Real.
In Roma he won 1 scudetto in 20 years, and that's it.
I want to say, I agree with you about some things which you said about Larsson (about staying in Celtic), but not all the people are the same.
Some will jump on every opportunity to play in a bigger club or to earn more money.
Some will stay loyal, even if that means less money, less titles, less flashy career.
You can't say with a 100% certainty that Larsson stayed in Celtic and picked an easier route.
Maybe he is "that" kind of a player who doesn't change clubs that easily.
And you know that he was like a God there.
God is even "too soft" phrase to express how fans adored him.
I know that you know, since you are from Scotland, but for other users, just listen to fans singing during a match:
https://www.youtube.com/watch?v=ia00uVJqC3w
https://www.youtube.com/watch?v=0f_Yb8vZGqs
Again, sometimes money isn't everything.
Even though, people like him are extremely rare...