Youtube's business model being questionable means that you think that it dubious at best and and the authority of the claim of its profitability is disputable. Eric Schmidt already said Youtube is very close to profitability, and should actually become a big revenue stream for Google in the coming years.
You need to have in mind that Youtube gets 2 billion views per day, and has advertisements directly on the content they serve. That's huge! I'd say that it's even bigger than Facebook's current advertisement model by a mile, even though Facebook obviously has a much higher number of views per day.
As the years roll by hardware, bandwidth, and manpower requirements for Youtube will invariably go down exponentially. If Youtube is close to being profitable now, in two years not only will it's revenue stream grow, but it's expenses will be diminishing also. Youtube was probably one of the smartest acquisitions on Google's portfolio, and it will no doubt be a source of revenue for them.
In any case, Youtube's profitability and sustainability might not be written on stone yet, but they have been far from being 'questionable' a long time ago.
The answer might seem obvious to you, but it's interesting to question non-the-less.
I for one, am not so sure their business model is sustainable.
My point was, it's a different question.