When I started to learn whistle by myself，i found some articles，they said ，in irish，people study whistles just for Higher Musical Instruments： irish flute ，and Uilleann pipes。
in traditional irish folk band，No special whistle player，Often is the others play it part-time。and if someone want to study irish flute，he often began from whistles for introduction。
Well I live in Ireland, play regularly in a large session and know lots of musicians so I think I can give you some input into your question. What you heard is correct to a certain extent but those articles you read must not have explained everything properly. Here in Ireland many children start off playing tin whistle in schools because for a start they are a cheap starting instrument, they introduce the child to basic scales and they are very easy to learn a simple tune on, so the child doesn't get bored when they realise they can play a tune very quickly.
It's true that most of these children will either stop playing music altogether or move to a different instrument entirely. However at that stage they will not have reached a high level of whistle playing, they will only have learned the basics. If they decide to move to a new instrument it's not because they are dismissing the whistle as a toy, they would be fully aware that they could continue on with it to a much higher skill level. Very few do because there's so many other choices of instruments used in sessions, so it's simply a matter of choice.
Of all the musicians I know over here in Ireland, I can't think of any that don't also know how to play a whistle to a certain degree, because they all learned it in school. However they wouldn't necessarily be much good at it because they never stuck with it.
It's very true though that knowing how to play a normal whistle will make it easier for you to also start playing low whistles, flutes and pipes if you so wish to expand. These other instruments are of course much harder to play so it's up to you if you want to take on the challenge. I started with normal whistle, then the low whistle and now I'm learning flute. Even if I ever do become any good on the flute that doesn't mean I'll stop playing the normal whistles, it will just be an extra instrument to bring to the session. It's a normal progression and I think you should also consider it. I think in time you'll find yourself wanting to try these other instruments and you might be surprised that they aren't as difficult as you first thought once you have that solid whistle background. I think it would be a shame to play only a whistle for 10 years of your life and not try another instrument that you might have been able to learn much more quickly because of it's similarities.