There is a good reason to pronounce it with long E. It is derived from the Latin word 'aether', from Greek 'aither', and 'ae' is a long vowel which always goes into English as a long E.
'Aether' meant extremely thin substance, and came into modern science meaning 2 things:
(a) A liquid that is on the verge of turning into a gas at room temperature. This name stuck on two chemicals, diethyl ether (the anaesthetic) and petroleum ether (very light petroleum distillates).
(b) The substance that was formerly thought to fill the universe and carry radio waves.
Ethernet is so named because it's like radio communication inside a cable, with carrier waves.
Now you know...