Does 筈はなくて grammar construction have a negative connotation?


I found the construction 筈はなくて and by helping me with the context, I suppose
to have properly translated but I am not sure.

> このバックパックは特殊な製法でできていて、見た目の大きさよりずっと多くのアイテムが収納できる魔法の袋…とまぁ、そんな都合のいいものの筈はなくて、  
>  This backpack has been done in a special way, it is a magical bag that you
> can put many more items than what appears to be possible to put in it ...
> but it does not turn to my advantage!
>
> 生地の限界がくれば溢れるし破れる。  
>  If it is filled beyond its limits, things come out and break.
>
> 勿論重さも健在だ。  
>  Of course, the important thing is that I am alive and well.
>
> 夢のような便利なアイテムはないんだよね。  
>  Yeah, is not an object of your dreams, it is not easy to handle!

Is it correct my interpretation of そんな都合のいいものの筈はなくて?

I think that the subject is denying the fact that the backpack is convenient.




The key here is the idea that the backpack is unusual, nearly magical, in how
much it can contain despite its apparent small size. The finishing phrase:

> そんな都合のいいものの筈はなくて

筈{はず} in this context refers to the speaker's _expectation_. The なくて here is a
conjugation of ない _"not"_ , so yes, this has a negative connotation. Much as
in English advertisements, this full phrase could be translated as _"there's
**no way** this could be so good!"_

Another more prosaic example of 筈 usage (usually written in kana) in a
positive context might be あの手紙はもう届いた **はず** だ。 → _"The **expectation** is that
that letter has already been delivered."_ (literally) → _"I'm **pretty sure**
that letter has been delivered by now."_ (more idiomatically)

