You say Ulissi, but he and Spilak are in different galaxies as far as results in GTs go. Again, I'm not even talking GC here, but look at partial results, stage wins. Ulissi has won 8 Giro stages, Spilak doesn't have a stage top10 in any GT. Not comparable at all, not even close. Also Spilak was a much better one week stage racer than Ulissi.Diego Ulissi is another. Competitive in just about any 1 week race around the calendar, but not a Top 20 finish in a GC to be found. Not quite as successful as Spilak, but all types of GC wins in week long races over his career.
There are tons of other riders who have been dominant in week long races who never won a GT, even if competitive. You’re right that Spilak is an extreme case though.
Remco looks great at the moment, but we haven’t seen him go over 2000+ over multiple climbs in a stage. Take Valverde, for example. He’s a GT winner, but never won the Tour. His biggest handicap over the years in the TDF and Giro is that he seems to have an internal altitude meter. He doesn’t ride as well above 2000 meters, which is why he is more suited for the Vuelta, where the most difficult climbs are steeper but at lower altitudes. Remco has never been tested on those stages with multiple big mountain passes.