- solve a new problem
- solve an existing problem better
- solve an existing problem cheaper
A new tech MAY allow that, and it MAY be used successfully toward that objective, but even in that case, that's hardly the core of it for most cases. Not saying it does not happen, but there are much more at play.
Then you either end up refactoring the whole setup for years (which usually is expensive, slows down the business development velocity) or rewriting it from scratch (or as a new implementation next to the old one).
If the original implementation would've been sanely made. Then making new features on top of it (or partial replacements or micro-services etc) wouldn't be that big issue. But usually these beasts are more like godzilla-level monoliths with bad architecture etc so it's probably easier to rewrite the whole thing.