Take away Pratchett and yeah that would probably drive it into being gritty grimdark, he was the driving force behind the tongue in cheek humor that made the game popular and then remained, sure it went a bit too silly with some of Ward's stuff that was more a parody of itself than the subtle blend of humor that Pratchett applied . So yeah remove Pratchett and we'd have a very different 40k.
I think Ward did some funny stuff; he did create one of the better comedy duos in the series with Marneus Calgar as the straight man and Cato Sicarius as the funny man. He did often go too far with them and the IG though and he was more of a fluff guy than a mechanics writer; 4E was not a good Edition for example. He did much better with a good co-writer to work out the crunch for 5E.