In terms of Dover-Calais, there will always be someone willing to take on the route. It's a simple, no-fuss operation with little complexity, especially if foot passengers are no longer taken. The Spirit of... vessels are purpose built for Dover-Calais too, so they're unlikely to sit around doing nothing.
Hull-Rotterdam and Hull-Zeebrugge are freight routes, and they do make plenty of money on the minicruises. I also wouldn't underestimate the popularity of both routes with bus tours.
The questionable ones are Cairnryan-Larne and Liverpool-Dublin. The first has quite a few disadvantages due to the location of Larne, and Stena are only 15 minutes longer into the centre of Belfast with easy onwards links to the rest of the island. Stena also heavily invested in this route in recent years. The latter is also questionable, as it just doesn't seem to have any real advantages beyond tourist traffic.
