I fairly doubt if such ICOs really require the amount they raised to come into operation. Most of the successful ICOs from 2017 still have 80-90% of fund raised on their one or more addresses. So before thinking of liquidity aspect, first important question is whether ICO has such large business that actually requires millions of dollars? The answer is no in most of the cases, that's why they are troubled by local authorities, tax authorities, etc. because they can't justify the use of funds and are forced to keep them as it is.
Else otherwise, market has instant liquidity of ETH. You can sell thousand a day without affecting market at all.
None of the ICOs give you a breakdown of how exactly they plan to spend the investment. Most of them are ERC-20 tokens and their products are supposed to be running on the Ethereum blockchain. This is part of the promise that Ethereum can become the "global decentralized computer" for development of Dapps. The scalability issues show that this is quite the pipe dream.
The problem with ICO's is not just liquidity but also the development. The products/ softwares they are supposed to be developing on a blockchain (whether their own or ETH based), actually don't have enough users and features to warrant the kind of Ethereum requirement that would justify their usage.
As part of self-regulation, if these ICOs want to be taken seriously, maybe they should really start giving out audited figures regarding their projected gas requirements for development, testing and usage before they become self-reliant.
For as long as they have been here, the requirement has never been rationalized. They just assume that it'll all be money when it really isn't that.