It seems like you’ve already answered your question if you consider yourself a sprinter and are getting good results at longer race durations.
4w/kg is a good level and you’re going to go up a hill at the same speed as another 4w/kg rider regardless of your/their physiology.
What do you mean when you say “at or below sweet-spot speeds, it seems I am able to better maintain intensity towards the end of the race.” ? You can maintain intensity better at the end than other riders? Or better than yourself at different intensities? Below sweet spot should be an intensity that can be maintained for a pretty decent duration, regardless of muscular build, since sweet spot is based on an individuals FTP.
There’s definitely value in being robust for the classics but that is more a question of parcours and terrain than endurance. A lot of GC and TT riders don’t handle the cobbles well (although there are exceptions). I wouldn’t be surprised if Kristoff’s FTP is higher than that of Dumoulin and Roglic.
Maybe you could share your power curve?