The argument doesn't really stack up because Spain had Bryan Gil, Pedri, Eric Garcia, Dani Olmo, Ferran Torres and Pedro Porro who could have all been playing for the U21s and they were all in the full squad instead. Added to Ansu Fati being injured and he's their best player. I'd suggest that lot is a bigger miss than England's? Also the future Spain team looks ridiculouly good.
A front line of Fati, Olmo, Ferran Torres, Gil and Pedri is a significant upgrade plus Eric Garcia has been excellent and would be straight in at CB .......
And they still won their group pretty comfortably.
I think you have to look at the resources used and how well they were applied. Eze, Jones, McNeil, Hudson-Odoi, Smith Rowe, Madueke, Greenwood ..... it's not like they were short of talent. This reads a lot stronger than Spain, Germany, Italy and Denmark's line ups.