The ending to the 1990 Stephen King has a known issue when it comes to writing endings for his stories. King can craft an amazing beginning and middle, then have the conclusion completely flame out. That's not to say most of his endings are bad, it's just that a lot of them don't quite live up the greatness of the story that proceeded them.
IT is one of those King books with an underwhelming ending, and it's largely due to just how epic the journey is getting there. Anything less than an amazing ending would've been a letdown, and the Losers' Club battling a giant spider, with some strange metaphysical stuff thrown in, certainly wasn't too impressive. When that's the blueprint writers are working from, it's no wonder both IT adaptations so far have had trouble sticking their landings.
It's hard to think of an idea that would've completely fixed the ending of IT 1990, but there's definitely a change the creative team easily could've made to make things more exciting. Notably, the makers of IT Chapter Two partially learned this lesson later on, although their ending struggled for its own reasons.
IT 1990: One Change That Could've Saved The Ending
The last 15 minutes or so of IT really suffers from losing the spark that powered the previous 2.75 hours, the spark in question being Tim Curry as Pennywise the Dancing Clown. Curry is the main reason the IT miniseries is as well-ed as it is, with the events onscreen instantly becoming more exciting every time Pennywise appears. Curry's performance is magnetic and captivating, and commands the attention of anyone watching. Up until the last act, it's never too long before Pennywise is back to menace a member of the Losers' Club, which masks a lot of the miniseries' other flaws.
While it's mostly true to the basics of King's book ending, IT 1990 losing Curry and gaining a ridiculous looking giant spider puppet is a huge downgrade. Without Curry in the mix, things become flat, and the bargain basement stop-motion for when the spider walks does the ending no favors. If IT 1990 had just found some way to keep Pennywise in the mix for longer, maybe only revealing the spider at the very end when it's time for the Losers to rip out IT's organs, the ending could've retained that same spark that Curry brought to his other scenes.
As mentioned above, IT Chapter Two actually took this idea to heart, having IT's spider form retain Pennywise on top of it, allowing Bill Skarsgard to continue his also quite terrific performance. This change didn't quite save Chapter Two's ending, but the sequel introduced its own problems, such as pointless jokes as the Losers fight for their lives, and having them literally talk IT to death.
