MrDenny was the only moderator involved in that decision. The others are just normal users. I saw the question and didn't think it was worthy of a mod-hammer, but he disagreed apparently.
There's really no way to phrase a question like that to make it acceptable for Server Fault. SF is for System Administrator (et al) only, this is a developer question to start with. Metrics that indicate scalability are not necessarily related to availability either. But both are a function of design decisions.
We used to commonly get questions from people (especially those developing the next Facebook killer, or GMail killer, or whatever large website killer) asking roughly the same grade of question. The short answer for all of them is that you should concentrate on making a functional web server first, then work on scalability.
That is not to say that building in a degree of scalability and availability, in the initial design, will not help in the long term. But far too many people become obsessed with making something perfect and lose sight of making anything at all.
As someone who is struggling figuring out where to measure the scalability or availability prospects of a particular design, you should probably simply concentrate on building something functional, and correcting the design as pain-point become obvious.