This isn’t strictly true - while the US did cut its order the war in Iran has demonstrated the issues with the USAF E-3 fleet and highlighted that a replacement is required.
Funding has since been secured for seven E-7 airframes, the two earlier test aeroplanes and an additional five.
As such, it wouldn’t be a great surprise to see this order grow further in future financial years; I’d argue it’s entirely reasonable that eventually the E-7 will fully replace the E-3.
There’s only so much you can do with a global-eye sized platform. Physics - radar size, power generation, and space available onboard all play a part in constraining what a global eye can do.