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They were designed for operating of US Carriers, to replace them you would have to design an aircraft from scratch and then fit the systems to it and be Carrier capable, a massive, difficult and expensive project. Still younger than the venerable C-130. Australia looked at the E-2 system, but fitted to the C-130J, eventually losing out to the E-7.Northrop Grumman started with the production of the first E-2D Advanced Hawkeye for the French Navy at the company’s St. Augustine manufacturing facility. The first of three ordered E-2D aircraft, is scheduled for delivery in 2027.
It is amazing that the E-2 is still in production 64 years after the first flight. This is also a good alternative for countries which are not able to acquire 737 AEW&C class of aircrafts, specially for C-130 users, because the E-2 also has Allison T56 engines.
France's First E-2D Advanced Hawkeye Enters Production - Naval News
Northrop Grumman celebrated the start of production of the first E-2D Advanced Hawkeye for the French Navy at the company’s St. Augustine manufacturing facility.
There are other AEW aircraft smaller & cheaper than the Boeing 737, based on business jets, e.g. Globaleye, Oron (G550 CAEW), & the Embraer/ELTA P600 AEW (which AFAIK hasn't actually been built). I think SAAB may still do conversions of stored SAAB 2000s, if you want something cheaper than Globaleye.
But for some reason, for some countries the E-2 is still attractive.
Taiwan doesn’t need the range offered by other platforms and I imagine runway requirements are less demanding. Not sure about pricing though.
Some countries have been reluctant to sell advanced weapons to Taiwan, for fear of annoying the Chinese government. And maybe the E-2D has a shorter take off run than some of the others, which may be important for Taiwan, given its only potential adversary. As John Fedup says, range is probably not very important for Taiwan: it has a small territory, & the E-2D has reasonable endurance at low speeds, which should be OK for coverage of Taiwan. It should be easier to integrate into its forces than other aircraft, too.