ExactlySolved, are you a Wolfram employee?
No, I'm an academic physicist. I use Mathematica everyday, and although I rely on the mathematical algorithms, I agree that other packages e.g. MATLAB and maple compare in this area (in fact I sometimes use MATLAB at the university for the excellent curve fitting toolbox).
The thing that I really admire about mathematica, that makes me go on and on about this product, is its structure as a programming language. By structure I mean the kinds of things that are done in an upper division undergraduate CS class "Concepts/Organization of Programming Languages" e.g. a book like this:
https://www.amazon.com/dp/0321330250/?tag=pfamazon01-20
Mathematica is a totally functional language like lisp or Haskell, which means that every expression has the uniform structure of a nested set of functions, and a program consists of a composition a functions applied to input to obtain an output. Mathematica's internal evaluations are ultimately rule-based pattern replacements of these functional expressions, a system which the user has direct access to. Everything in mathematica is an expression, even input and output (arbitrary graphics are expressions, not side effects, even the notebooks themselves are expressions).
So that's it, organization and consistency are why I like Mathematica.
I found this to be rather amusing.
The reason that sum^(-1.5) fails the ratio test, which is a bug, is (I'm guessing) that there is mathematica code which compares 1. === 1 and yields false: the use of a machine number exponent leads to the 1. , and the ratio test fails so the sum is reported as diverging. Using n^(-3/2) works, but sum n^(-2.) fails, I'm pretty sure that the comparison of a machine integer and an exact number is the reason.
Will the engine to the site ever become a part of Mathematica itself, such that people can set up wolfram-alpha-like natural-language web interfaces to Mathematica scripts or knowledge bases they created themselves?
Yes, W|A will become part of Mathematica, using internet functionality to reason live results from W|A inside a notebook. Also relevant to your question are the soon to be released W|A APIs, best suited for Mathematica itself but also availible for other major web platforms. I would not suggest, however, being overly optimistic about 'openess' of these APIs.
Also I share your concerns about Mathematica scaling performance-wise to this load. The only reason I still use languages like C for some tasks is speed, and I'll readily admit that Mathematica is 10-1000 times slower, having an interpretive kernel. Hopefully computer performance will increase enough overthe next few years to realize more ambitous uses of W|A.