I'm sure Heir to the Empire sold really well... 22 years ago; but I think that was mainly because it was introduced into a starving market that hadn't had any new Star Wars media for 6-7 years (outside of tabletop RPGs, which are always going to be an extreme niche market). So, the book naturally reached bestseller status, but I don't think the Thrawn trilogy ever reached worldwide phenomenon status. At least not in the way that more recent non-SW book series' have (for example, the Harry Potter books, the Twilight Saga, etc.).
HTTE is currently the #2 ranked Best Selling Kindle Star Wars book, behind only The Last Jedi which came out 3 weeks ago. In Kindle > Sci-Fi > Space Opera, HTTE ranks #36 while The Last Jedi ranks #22, and in all Books > Sci-Fi > Space Opera, it's #40 and #24 respectively. HTTE is still big with a current audience, as big as any Star Wars book and second only to the newest SW book, I think that qualifies it as a phenomenon as much as any Star Wars book can claim.
Darth Vader is becoming the Mickey Mouse of Star Wars.
"In Brooklyn, a castle, is where dwell I"
The use of a lightsaber does not make one a Jedi, it is the ability to not use it.