So, "One Search to Rule Them All" is an impossible pipe dream. Or is it?
That depends on what you mean. If you mean "one search engine that will replace all the others and allow us to never use anything else", then yes, it's impossible. As impossible as the ball that can be used for every sport.
But if your goal is to build a search engine that can do a simple keyword search on the most common content types... then the answer is a resounding maybe. And given the demand from the public (because who wouldn't want a magic djinni that would do all our academic research for us?), the quest began for the holy grail of library science: a simple search interface for all library content.
I don't want to be too negative. A lot of progress has been made, and we've learned a lot about how to improve search engines in general. And after twenty years of development we've finally got some options that don't suck!