No you couldn't have. At 17 years old Bellingham wasn't going to be guaranteed first team football at Utd. Furthermore Utd have been a madhouse of a football club, chopping and changing between different coaches, set-ups and ways of playing. Any young player would have been mad to take any assurances from Utd, not knowing if the person that gave those assurances would be at the club in 6 months time.
Unlike at Utd, Dortmund could both offer him first team football from an early stage and the stability that didn't just guarantee that on day 1 but also 6 months down the line and beyond. There's a proper structure in place at Dortmund so that if the coach changes, they're not ripping things up and starting again, leaving players as collateral damage.