Comparing Cobo to Kuss and Valverde to van Aert in terms of GC racing. I don't see it. Distinct difference.
(at the time):
Valverde had won the 2009 Vuelta and not gone for a GT GC since.
Cobo was the defending Vuelta winner.
Should they have told Valverde to ride for Cobo because Cobo was the more recent GT winner?
At the moment, Kuss is a one-hit wonder, a fluke GT winner who got that title because he was underestimated from a break Giovannetti-style, and then the team panicked at the burst of negative PR that came from allowing the other riders to race it out, and told them to knock it off and let Kuss win the race once they'd eliminated all other teams from contention. Since then, Kuss has had a pretty tough season until just recently, and has had illness problems and struggled to assert himself as a legit leader as a result.
He may back it up and prove himself to be very much a worthy GT leader who deserves ongoing support, but until he does, it makes total sense for Visma to have back-up plans. Remember Ryder Hesjedal defending the Giro, being an irrelevance by the end of week 1 and DNSing halfway through week 2? Movistar's attempt to treat David Arroyo as a legit GT GC rider after his surprise podium thanks to the break in 2010, where he got chewed out and spat out back to the 10th-15th range where he'd always been at the race? For three years after winning the Giro, Tao Geogeghan Hart's best WT stage race results were an 8th and a 10th at the Dauphiné. Hugh Carthy got that Vuelta podium but since then has settled largely in bottom-of-the-top-10 anonymity. Kuss could be another Tony Rominger or Primož Roglič, sure, but he could just as easily be one of those guys I mentioned.
Kuss didn't win the Vuelta at age 23 or something that makes them think they have the new hottest commodity in town or anything either. He's going to turn 30 next month. It's totally reasonable to have backup plans in force.