The Thrawn trilogy was very good and easily the best part of the old EU (which had some excellent stuff scattered about, but was also frequently terrible) -- and much better than the sequel trilogy -- but trying to adapt it into a new movie trilogy wouldn't work at this point.
If you take out the OT characters, you're essentially creating a new story entirely. It's no longer even remotely similar to the Thrawn trilogy because they were integral to nearly everything that happens; you can't just sub them out with different characters and have the story work as-is.
If you don't take them out, you still have to massively overhaul the story because it doesn't work with them in their 50s and 60s (or older). The longer time skip would cause other problems like the plot making less sense 25-30 years after ROTJ instead of 5, and also causes issues for characters like Mara Jade. Plus, some of the plot points were essentially eliminated by the prequel trilogy.