For example, if we went back in time, would all of the core functions have been implemented this way? Would this be a possible drop in replacement in the future? Could future versions of Clojure integrate these ideas more deeply? If so, what are these backwards compatibility concerns?
I don't think so - the existing implementations work on the higher-level abstraction of sequences. Reducers are optimized parallel versions that work on collections. While parallelism is extremely useful in some parts of your code, there is overhead and I don't think you would want either the overhead or the restriction of working below the sequence abstraction in the general case.
I seem to see some of the same choices being made available in the new Java 8 collections and parallel operations work. That is, it is up to the developer when to "go parallel".
For an entirely different approach, check out Guy Steele's Fortress language which eagerly executes most things in parallel by default (like all iterations of a loop, arguments to a function, etc) and you have to tell it not to do that.
Guy's Strange Loop 2010 talk is an interesting complement to this work: http://www.infoq.com/presentations/Thinking-Parallel-Program...
2) If seq or first is called on a reducible, wouldn't it be easy to just implicitly realize the reducible in to a sequence first?
2) That's possible, but it makes it too easy to write code with abysmal performance because of (1). The common case is that you call both first and rest on the reducible. If both turn the reducible into a seq first, then both will take O(n) time in the best case (might be much worse depending on how the reducible was built up). Combine that with the fact that most times, you're going to recurse on the rest, and you've got an O(n^2) algorithm where you expected O(n), if everything is based on reducibles. Additionally, it's impossible to take the first or rest of an infinite reducible (well, perhaps you could do it with exceptions -- in general you can turn a reducible into a seq with continuations).