I can't speak for The Sun unfortunately, but I'd love it if they snapped up Horace. This is very sad and disappointing news, and yet another very significant nail in the coffin of newspaper cartooning.
I also think it is totally misguided, but the unfortunate reality seems to be that people who love great cartoons, as Horace undoubtedly is, appear to be the least likely to complain when they are dropped. I'd suggest what is needed therefore is a coordinated campaign using social media to find and encourage Horace fans to speak out loudly through whatever means available (and there are quite a few), letting the paper know in no uncertain terms how much the strip is missed (this would have to be timed for after the strip stops appearing, unless the paper announces it ahead of that time - unlikely).
Editors do listen when their readers speak, sometimes even when it's only a few. I personally witnessed the resurrection of Biffo The Bear in The Beano back in the 70s (when it had a bigger circulation than most newspapers have now), after the editor took just one call from a long-time reader...in his 50s as I recall, tearfully pleading for the recently dropped Biffo to return. Roger will know that the Editor concerned, one Harold Crammond, was no soft touch, but immediately reinstated the Bear to the comic. Different times perhaps, but voices do get listened to.
Unfortunately, due to my association with a rival newspaper, I'm not really in a position to kick off such a campaign, but if I was, I'd be scouring the web for any groups or forums with Mirror connections and imploring anyone who feels this to be the misguided error it certainly is, to make their voices heard at the very top. Newspaper switchboards are usually very accommodating when it comes to imparting contact details, even for the editor.
Shame I can't get involved. I wonder where we might find anyone who can...