You know Cankles Clinton is going to say the emails were unfortunately destroyed. Why not subpoena the people who received her emails or sent her emails? They would have the same emails on their servers.
The only problem would be identifying who those recipients/senders are, but a smart investigator could come up with a list of the most likely ones pretty easily.
Because her foreign donors are unlikely to cooperate under your idea. Trading on treason is not about to be captured on a server that can be investigated by those pesky Republicans. Besides, those who received her emails on their government server are captured, but the subject matter is likely Chelsea, baby and fundraisers.
Hillary is bought. Her activity is money. She has no intention of betraying her foreign sources for obtaining money. She intended to be president. They intend for her to be president. They invested in her and her presidency.