If he joined Jumbo or UAE, given Vingegaard and Pogačar's current form, he wouldn't be able to replace them as the team's lead rider—not in one or two years, maybe not even in three or four.
Besides, his path to improvement relied on earning UCI points. If he were constantly sidelined by the team and couldn't even get into major races, he'd never earn enough points to break into the core roster.
At his current level, making significant gains in power and other metrics through routine training alone was already extremely difficult.
He simply couldn't afford to wait for Vingegaard and Pogačar to ride out their prime years before taking one of their spots to become the new king of the road.
That's why, while he couldn't say he'd never join Jumbo or UAE in the future, he certainly wouldn't join them now, having just reached the World Tour level.
