Thanks for the kind words, Solesky.
I understand the question and thanks for asking it, it's a good one. Hopefully I can explain my thoughts just as clearly.
I think that depends on the starting point and the potential of the trainee a little. I think it is very possible for players to make both sides, though, but you're right, if you look at the really big sides they have players who peak later but with much higher secondaries. This is much easier to do if you spend at least a full season in the teenage years pumping one on one or secondaries.
If the trainee is MVP or lower this is less of a consideration, because they are less likely to get to those monster levels, so it makes sense for the manager to get the reward of getting their player up to U21 level to make the team and get the rewards of that. In the same way, if you're lucky enough to draft a high TSP HOF or ATG at 18 you can easily train for both, but I think with a lower TSP higher potential guy you can end up with a better build by spending a bit longer rounding them out.
But Whitewind does excellent work advising managers of under-21 players so typically when young guys graduate out of that program they have a sound base that can be built on to make a NT player - the challenge often for managers seems to be how they manage their finances to get there - which leads to an approach I've developed.
I've kind of made a point with managers of not demanding they train certain things but rather give my suggestions about what the team might be looking for when they do eventually make the side and how they might get there. I understand that not every manager is training players purely for the NT but rather to help their own sides grow, so I try to ask managers what they are looking to do with their players and then see how they fit with the other resources we have coming through so I can somehow end up with the best balance and depth possible long term. I have a fairly massive spreadsheet system for tracking how potential players at each age are progressing that allows me to filter by skills to keep tabs on what depth we have coming through - my goal is to end up with a squad that has the best overall range of skills to compete with those bigger sides, which is not always easy.