This sale has been mooted/rumoured-about for a couple of years. It is a scenario that I had hoped would not come to pass.
Norfolk Line's Dover-Dunkerque service offers by far the best-value fares on the short crossing - especially for ship-spotters! (better day and short-break fares and much better photo-opportunities at the French port, for a start . . . and then a half-hour shorter drive to Terneuzen!).
I am not a big fan of DFDS Seaways because, after taking them over, they closed ALL the ferry services that I found their most useful and, indeed, sometimes needed: Harwich-Bremerhaven; Harwich-Hamburg and Newcastle-Stavanger. And one thing DFDS has never been is CHEAP!
However there is a hope that they will realise that the only way for the Dover-Dunkerque run to remain viable is for it to compensate for the longer crossing time (compared to Calais) with more-competitive fares.