At the end of the day, ABC’s IT wasn’t an exact adaptation of the novel from page to screen.
Of course, fans of the IT novel can’t complain that the reveal of Pennywise’s natural form as a giant spider didn’t happen on the page.
That said, Tim Curry captured what made Pennywise so scary in the source material, and we can be grateful that he was part of the project, even if he’s not a fan of the ending.
Stephen King wrote the twist into his book, although the miniseries did take some liberties with the other events down in the creature’s lair in the climax.