The thing you forget about "sensible people" is that there are "non-sensible" people born every minute, or come into this space. I argue that my estimate of 2 to 3 years is conservative and as long as there are people, as long as they think "so, there's a chance" ... ICOs or some variation of them will continue to exist.
That may be true that some variation of the ICO market will stick around indefinitely. But there are only so many times that you can continue to say things like "ETH is Bitcoin 2.0!"
"Oh no wait, not ETH, now it will be XXcoin that is the next Bitcoin 2.0!"
"Oh no wait, it's not XXcoin, it's going to be XYcoin that is the next Bitcoin 2.0!"
"Oh no wait, not that XYcoin stuff, it's YYcoin that is the next Bitcoin 2.0!"
... and on and on...
After a while, people will start to see the ever revolving door of the perpetual scam that is "This is the next Bitcoin! Trust me!"