Yes we need all of it. Until we can get a handle on who has had it, who has it now and then trace the contacts of everyone who is positive we're stuck in some degree of isolation. Sending people running around as soon as the curve is down without this in place ensures it pops back up.
We get one shot at this, and the idiots in charge don't give me much faith that it'll be executed correctly.
I haven't read the WHO thing you initially responded to but I'm definitely not suggesting that should be the case. At a minimum I think we'll need 3 weeks of the lockdown we currently have from the point we start to see a sustained decline.
If we follow Italy/Spain etc, then that should start at some point next week and run through to the 5th week of isolation (which here would be the week commencing April 20th).
Let's just say, hypothetically, that happens. So we do another 3 weeks of this lock down from April 20th. From May 11th, we then - as Denmark / Austria are planning to do - gradually ease certain measures as long as the numbers have continued their decline over the previous three weeks. Given Europe has a 'head start' on us, we can also use them as a canary. For example, if Austria's numbers start to spike again a week after they've eased lockdown measures, we'll know we'll have to stay in this lockdown for longer before gradually easing off (so instead of May 11th, it could be May 18th, for example).
Then it'll be a case of slowly and gradually having more things return while at the same time still using social distancing measures, still demanding home working where possible (personally I'd be bringing in measures to encourage that across the board long term as it's just quite a good solution to a lot of issues in general) etc.
That would, in this theoretical scenario, probably place us in mid-to-late June where we could have some real semblance of normality. And perhaps we could say still ban large gatherings until the end of June / first week of July for example.