A) The evidence is in the article. It's heading towards a small number very quickly.
B) Everyone is learning to speak English. Everyone in W. Europe under 30 speaks English fairly well. It's happening in M/E and Asia as well.
Once immigrants to W. Europe who speak English + some foreign language can get services in English - there's no point in learning the local language.
There are only 12 million Swedes. 10% of them barely speak Swedish - and the number is growing rapidly.
Once young people speak English fluently and services are in English - so much work will be in English ... Swedish loses all real utility. And utility is an important thing.
'Long term' I think a diaspora of languages is important, but people make short term decisions: 'what is important in my life'?
I fear in 20 years, in Sweden, young people will equate 'Swedish' with 'old, out of touch, nationalists and racists' - which is an unfair characterization but the perception is already developing: young urbanites speak English fluently, rural, less developed communities, less so.