The idiom meaning soon to happen or appear was originally coming down the pike, not coming down the pipe, but both forms are now widely used and understood.
In coming down the pike, the noun pike is short for turnpike, which is a broad road, sometimes a toll road. This usage of pike originated in the U.S. in the early 19th century, and the earliest known instances of coming down the pike appeared around 1900. Pike soon fell out of use and has survived almost exclusively in this idiom, so it’s understandable that so many English speakers resist using it. Meanwhile, pipe is of course a very familiar word, and things do come down pipes sometimes, so it’s easy to see why pipe has taken pike‘s place in the idiom, even if the pipe metaphor doesn’t hold up under logical scrutiny.
In current searchable news publications, down the pipe appears once for every two instances of down the pike (though some instances of down the pipe have to do with plumbing). Examples such as these are easy to find:
Luck is the all-world, can’t miss, best quarterback prospect to be coming down the pipe in decades. [Toronto Sun]
More austerity coming down the pipe doesn’t bode well in the months ahead. [Business Insider]
But examples like these are about twice as common:
A showdown over the debt limit is coming down the pike. [NY Times Economix blog]
An important aspect of succeeding in business is knowing what the competition is up to and what’s coming down the pike. [Washington Post Capital Business blog]
The next Republican hostage-scenario is coming down the pike. [truthout]