It's all come from a rather silly example that really won't happen. Januzaj wouldn't even be eligible to apply for citizenship until 2018, assuming he remains in the country.
No way is a talented player with much stronger ties to other nations going to hang about until he's 23 on the off chance that he gets picked for a country that he has no real connection with.
Also, Belgium are a pretty fucking good team. He should pick them.
The argument in general is much more difficult though. I think if you've grown up here then I think you have every right to be proud of being British and playing for England. If, however you came here later on in life to play football, then you shouldn't be eligible.
I don't think many people would say John Barnes should never have played for England. A good current example is Saido Berahino who plays for the mighty Baggies and is also banging the goals in for England U21's. Born in Burundi, he came here when he was 10 (earlier in his life than Barnes) and I have no doubt that he's a proud Brit.
I reckon there should be 3 ways to qualify:
1) Born in England
2) At least one English parent
3) Lived here before turning 15 and with British citizenship
Also, if you are a citizen of that country (usually occurs after 10 years & a test).
It's generally 5 years and the test only applies to non EU citizens.