(Edit: CPH-Flyer explained this above, so I've edited out some repetition.)
Surely when there was three classes on domestic/European routes, business would sometimes be one empty row? (At least domestically.) Plus was introduced in 2013 if memory served me, and replaced both business and economy extra, so technically Plus and business never co-existed on domestic/European routes. And the reason business had so few passengers was that many "domestic" companies' travel policies did not allow business class travel on short haul. That is unlikely to have changed in the 11 years since, which is why I always thought the re-introduction of business class was unlikely. Now that we've been proven wrong we'll see if code-share and business leisure will be enough on European routes to justify the re-introduction. If prices are much higher than Plus, I have a bit of fear that SAS may have misread their home markets on this, which may cost both revenue and customer base.