Not sure how we can for-sure say Lesnar draws an audience. He's only had 2 UFC fights.
UFC 81 (Lesnar vs. Mir) did 650,000 buys. More people tuned in to see Lesnar fight *in his first UFC fight* than for any single Randy Couture fight in his entire UFC career.
Couture, in his entire UFC career, never reached 600,000 buys for an event that he headlined. And he fought Liddell three times, plus Ortiz. The third Liddell-Couture fight only drew 410,000, and it had the benefit of happening during the start of the MMA boom, plus the first Ultimate Fighter to promote it. Couture's biggest buyrate ever was 534,000 for the Sylvia title win.
UFC 87 (Lesnar-Herring) did over 600,000 buys as well, and that was after people had seen Lesnar get beaten by Mir.
Additionally, pretty much every MMA website reports record traffic each time Brock Lesnar fights. This all can't be a coincidence. Clearly, people are interested in seeing Brock Lesnar fight.
So you want them to cancel a title fight, which they built their reality TV show around this year, because Randy came back? That'd be pointless.
This works out fine. People complain that Brock is getting a shot too early, yet they have no problem with Mir getting a shot at the interim title after beating Antoni Hardonk (please) and Brock himself.
People also have no problem with Randy Couture being champion, even though he hasn't fought since August of 07.
There is NO other choice for the title shot right now.