Han Shot First wrote...
Indeed. Liara's development from naive archeologist to intel broker is already a hard one, but still possible. From intel broker to hard-boiled Shadowbroker? Nah. Doesn't fit. It doesn't work for me, not at all.
But I accepted it - Liara seems to have a huge fanbase and half of BW-devs seems to be Liara fans as well. That's why we have "Lair of the Shadowbroker" but no "The Virmire Survivor" DLC 
Just out of curiosity, why did you find Liara's progression from naive archaeologist to intel broker more plausible than her transition from intel broker to Shadowbroker?
More than one reason:
- you need to read from "archeologist to Shadowbroker"
- and even if you don't read it like that - if it's that easy to a) find out where SB has its lair and

become the SB once you dispatched it, why did the yahg succeed as the SB for 60 years? I mean Liara is not dumb, but she's also not the most intelligent person. She is, as I said, a tad naive, that alone disqualifies her as a intel broker. To be a good intel broker you already need to have some balls (so to speak). To be the SB you need more than that.
The reason why I ask is that the Shadowbroker is an intel broker. The Shadowbroker just happens to be the most powerful and influential intel broker in the galaxy. From ME1 to ME2 Liara has changed careers, while she hasn't from ME2 to LotSB. In LotSB she's just moved up to the most elite level of her ME2 career path.
Lets say I'm a boss of a small corp division. Do you think I can run the entire business? Nope.
Being the SB means you need to be able to pull strings, you need to have an overview over millions of incoming datasets, sort them and use them. It's already quite implausible ONE man / woman can do that ... even not if s/he is experienced in that business. Liara is not experienced as a quite young intel broker and she never displayed more capabilities (multitasking) than any other character. So basically, Shepard could be the SB as well ...
Biotics are common in Asari society, so she's not that special. So the writers decided to make her the new Shadowbroker ... and never took time to explain why.
Perhaps Benezia was an intel broker?
I can't even remember if that's in the lore, but just in case it was ...
That might explain Liara's career shift as well as Benezia's pre-indoctrination connection to Saren. Mom dies, and following Shepard's death Liara accesses Benezia's files and picks up the pieces of her organization.
Or at least that is the route I'd go if I had to come up with a backstory to explain Liara's sudden shift in careers.
... it still doesn't fully explain why Liara changed her career. I mean, she's intelligent, but she's still a naive young woman (by Asari standards). Not a good base for being an intel broker, as I said before.