Heaven's Feel kinda has to hinge on the romance with Sakura since its about Shirou picking the one over the many. Saber pretty much shares Shirou's ideals to an extent and Rindoesn't think to make Shirou change so drastically. Been a few years since I last read it myself but the Sakura situation requires Shirou to lose Saber, his chance at winning the HGW and learning about Sakura's past to make him willing to give up his ideals to save just the one person. HF also gives us some of the sickest moments in the franchise.
Edit: Its one of the reasons why I like Miyu!Shirou since his goals are just as relatable as HF Shirou but we don't need all the awful H scenes. You'd think Nasu woud get a bit better after so many?
Oh no, I agree Sakura and Angra Mainyu just nomming everyone was annoying. The previous mid/end bosses didn't need to job just to hype up the new ones.
The romance as the reason for HF is a pretty poor. Learning Illya was Kiritsugu's daughter and his sister would have been a far better reason since she is the last connection he really has to his savior and hero.
Also Illya route when? Prillya is fun but OG Illya is still best.