Well, doing over time means you can steer (i.e. adapt to new situations and learning). Among the problems with Sri Lanka's decree is there was no time to adapt.
For example, it takes time to switch a field over to organic, and who will pay for the more expensive yet not yet qualified organic crop? Where are the distribution chains (never mind the supply chains).
I don't know enough about Canada's plan (only this article) to criticize or praise it. But I do know that your criticism, on its face, is not appropriate.
I think it is different. One thing is banning immediately all fertilizers and tell people "we just grow everything organic way, but you should figure out how... Now". And another thing is reducing their quantity so that farmers adapt by learning techniques to optimize the growth and (maybe) they'll start producing more compost to compensate deficit of 'classic' fertilizer