At some point you'll have to decide: "enough" for whom? For someone just trying to accumulate some money for retirement, even hour-level liquidity probably looks like more than is useful (and will his broker even act on a request he makes in less than an hour? maybe someone else can answer that one).

It depends on how you draw the boundaries for an 'entity'. A sufficiently large company may have many departments trading in different ways fairly independently. If you are going to tell people, "You can't sell/buy sooner than X seconds after you buy/sell.", then large organizations will have a problem being nimble being hindered by regulations about how soon you can flatten a position. Even brokers like Ameritrade and e*trade will have problems acting on behalf of their clients because different individual client will want to trade differently, if you treat them as single organizations. What about dark pools? Are you going to regulate those as well? The trading venues are about people trading stuff. It's people acting independently out of free will.