I think you have to look at all the different factors that come into play when young players are making contract decisions.
Some the incumbent club have control over and some they don't.
Off the top of my head and in no particular order:
1. Development opportunity
2. First team opportunity
3. Facilities
4. Other players
5. Current success of club
6. History of club
7. Location/geography/climate
8. Agent influence
9. Language
10. Family support/advice
11. Desire to travel
12. Money
13. Regular game time
14. Managerial pull (Fergie factor)
15. Managerial longevity
16. Cultural fit
17. Positive relationships with coaching staff
18. Length of current contract
Plus one or two others I may have missed.
The order/priority of these factors will dictate a young players decision. (Or any player for that matter)
You could argue that a handful of top clubs, United included, have good development opportunities, facilities, other great players, historical and current success. So they even themselves out.
So the other factors become deal breakers.
We will never compete on the climate in Spain or Italy, or even with the attractions of say London. There's is not a lot we can do about that.
So we have to do our best around the remaining areas. Foreign imports usually have an agent in situ, have already moved country, are used to living away from home, have learned English...so it is much easier to do it again. For English based players (who have been limited by 90 minute rule) it would be a much harder decision to move country which is why you don't see it happening. Also there is plenty of good money to be made playing in England so that factor doesn't really come into play.
We have the best track record of giving players a chance of first team football, something even Pique acknowledged when he left Barcelona, and we try to maximise that advantage...or at least I know Ferguson did. Scott Wootton said the same about Liverpool who have a poor historical record...while Newcastle are terrible.
I speak to many parents on a Saturday morning and they say that they rejected Chelsea and City because their kids just won't get a chance. Arsenal, Palace, Everton and Southampton are also very good.
How we put the whole package together makes a big difference.
Having lost Ferguson through retirement perhaps some players are less likely to come/stay. But given time, Moyes longevity may be attractive. Players like consistency at the top.
Pique, Rossi, Pogba, Morrison, Shawcross and others all left for very different reasons. But if you look at other clubs, Barca and Madrid included, they lose talented youngsters for a variety of these factors too. It just isn't in our face every five minutes like the current Januzaj situation.
Finally, whether we like it or not, we have a wage structure in place for a reason.
We can argue about it all day long but it still exists for a reason.
If someone wants more money, then sometimes you just have to say no. It could be a annual salary which will upset all the other players (fine with an exceptional player like Messi or Ronaldo but not an inexperienced 18 year-old)...an outrageous one off payment to an agent...or some other financial request.
As with 'guaranteed' minimum number of games which is what Pogba's agent wanted, I understand fully why Fergie rejected that idea straight away. He couldn't even guarantee Rooney or RvP a certain number of games.
I think the whole situation has blown up in the media because we lost Morrison and Pogba very quickly and they were clearly talented. But both left for different reasons, many outside of the clubs control...regardless of what some people think on this Forum.
Let's hope that Januzaj's 'list of factors' can be met by United and he therefore signs a long-term contract with the club.