"Behind every great fortune there is a crime."
I saved thieves for last among the "Core Four" human classes, since he's a bit unusual. For one thing, he reaches name level sooner than any other class (160,000 xp). For another, he doesn't build a stronghold, tower, or castle. He usually sets up shop in an established area. Lastly, exactly how his endgame plays out can vary a lot more than most other classes, depending upon the DM and how the player wants to handle things.
A ninth level thief is not actually all that powerful a PC. With an average of only 18 hit points, limited armor options, and no spells he is not making anyone quake in their boots.Of course he has probably picked up some magic items along the way and made enough money that he owns quality gear, but he still isn't all that intimidating on his own. A name level thief's strengths lie in his ability to operate "off radar." The underground world of crime and corruption is bread and drink to the higher level thief. Dark alleys and shadowy corners are fine for a low level cutpurse or thug, but master thieves need to think bigger.
I've always thought Charisma should have been a prime requisite for thieves. Sure, DEX is nice, but eventually being able to convince and persuade is going to count for a lot more potentially. Ah well, a topic for another time.
Cook says that name level thieves "...a thief may construct a hideout (a fortified house in a city, a cave network, or so forth). A thief who has constructed a hideout will attract 2-12 1st level thieves who have come to learn under a master."
2d6 apprentices is not a lot to work with, manpower wise. However, he's not manning a castle or patrolling a barony, he running a gang of crooks. Sure the gang might one day rule a whole city's criminal underworld, but that's not something most DMs would just hand wave away. They'd play that arc out (as well they should!).
Furthermore, thieves don't need to set up shop in a city. They can be highwaymen, smugglers, spies, or pirates. The Master Thief can arguably adapt to settings or individual player concepts to what they want more so than the other classes.
To use the pirate example, a sailing ship costs much less than our Tarnskeep example. A small ship might even be crewed by your 2d6 apprentices alone. Not to mention it provides convenient transportation to various parts of the world for the PCs and the thief can sack ships or raid coastal settlements as they go.
Name level thieves need to be smart more than tough, and willing to look at the different ways they can profit from their newfound status in the shady underworld of the setting.