San Remo has always been prestigious, but it also depends on the era. Suddenly, Liège-Bastogne-Liège isn´t important race, while San Remo is everything. When San Remo was won by sprinters in a massive sprint, it suffered a crisis of popularity and was a heavily criticized race.
Historically, it has always had more prestige, yes. But I think that now Liège-Bastogne-Liège is being undervalued and San Remo is being over hyped because of what has happened recently in both races.
The Vuelta is a race that even Indurain, being Spanish, didn't care about. Merckx went to win it because he was paidan indecent amount of millions at the time and because they designed the route with him in order to better reach the other objectives.
I mean, no cyclist in history has given any importance to the Vuelta, not even the Spaniard biggest star.
That the World Championship is a thousand times more important than the Vuelta is indisputable.