Note that Addabox stated "appears to be". He didn't state that a successful 7" device wasn't possible from Apple or anyone else. He even backed up his comment with evidence to support the current trend. The iPad is the most successful tablet on the market. There are plenty of other non-cheap 7" tablets that simply aren't making a dent.
The only tablet that is making any headway is the loss-leader Kindle Fire which is really a glorified eReader at this point.. Despite it's gain in the "tablet" market and it's retail price that is only 24 to 40% of an iPad it still sold well below the nearly year old iPad this past holiday quarter. I don't know of any supply issues that kept the sales of the Kindle Fire down.
Bottom line: So far there is no relevant tablet market, there is only an iPad market.
Nokia is a great example of thinking that creating an ostensibly complete product line that overs all HW needs is the way to ensure success.
PS: Also note the netbook market. Remember analysts saying that Apple needs to enter this market or risk losing what little market share they do have. They never touched it. They could have offered the best netbook on the planet but the didn't touch it. Where they smart to do so? I think so.
I like your bottom line (Bottom line: So far there is no relevant tablet market, there is only an iPad market.) Still I believe that the reason for not having a 7" market is that there is no iPad there.
I dont't understand what you mean with your statement about Nokia, to me they are an example of where you end when you ask the consumer market what to do, instead of presenting the solution and being ahead.
About the netbook market, i totally agree, a quality supplier should never pursue a race to the bottom.