Sublime Text will never be a plausible replacement for Emacs as its design does not encompass all of possibilities a lisp machine provides. Sublime Text is a plausible replacement for any tool which only targets text editing though. It has many converts or new users whose use case is only a text editor though, so to that extent Sublime Text has been successful.
I don't understand what you mean. ST3 is scripted in Python. If Python was as pervasive in ST3 as elisp is in Emacs, and provided similar concepts (buffers, regions, markers, modes, hooks, advices...), why couldn't it "replace"[1] Emacs?
[1] replace here means "being as powerful as", not actually converting Emacs users, most which are happy with Emacs..
In answering this I will ignore details specific to the two tools (such as how the implementations are modified), the merits of either, how they are used, and only compare their design goals.
It is true that both tools are mutable and that hypothetically their potential use cases will overlap entirely.
The reason this will not occur though is a consequence of one tool aiming to provide a general environment by design and the other tool aiming to provide a general environment for a specific use.
Less abstractly, consider the difference between having a factory which produces a specific item and having a factory which can produce any factory.
Possibly the specific item the factory produces is sufficient for the recipients of the specific item, and further that specific item may even make other items the recipient possessed moot.
However, the factory which can produce any factory can still (and possibly already has done so) produce the specific item.
Emacs' design (and Light Table's) is much more similar in breadth to the factory which produces other factories.
Sublime Text is more similar in scope to a factory which produces a single item.
The difference in design choice leads to Sublime Text and other text editors being comparable in features and it is also why Sublime Text can replace Emacs' text editing functions.
At the same time the design choice also leads to Emacs being a RSS client but Sublime Text not also having that functionality (or being reasonable to expect), despite the potential for the functionality to exist in both tools.
The difference between the two goals is why Light Table is much more intriguing to me than Sublime Text is despite the utility Sublime Text provides and how well executed Sublime Text is.
