I'm being a little pedantic here, but it's not exactly true that Equihash is ASIC-proof. Someone could theoretically take an existing GPU design and strip out the circuits that aren't relevant to general GPU computing to develop a functional ASIC for the task. However, that's an extremely expensive proposition, and it's incredibly unlikely given Ethereum's plan to move to Proof of Stake. By the time an ASIC rolled around to manufacturing, it would already be obsolete.