Hashing power is what mines the blocks. There will be no "voting the issue out" if the dominant chain is using Segwit. Just because you buy a few BTC on GDAX or something doesn't somehow give you more say than the miners over what software gets run on their own machines. Vast amounts of hashing power is concentrated in the mining pools that are signaling support for this and 99% of the people speculating on these currencies don't care about the underlying protocol, they just want to make money.