It’s probably more accurate to say that the haste will lower the duration until you earn a whole additional tick, at which point the duration will go back up again (but with more damage / healing because of that extra tick). Getting more haste will never be a bad thing, but there will be break points where haste is more valuable than others. You should also never want to cancel the spell early.

This is definitely one of those cases where we’ll have to see how it feels, but it is more intuitive when you’re actually casting spells in the game than it might sound on paper. (Source)

[...] The answer we were trying to give was that “channels won’t just get shorter and shorter the more haste you get.” But a more complete answer is that hasted channels will get shorter until you get a whole additional tick, at which point the duration will go back up again. We don’t want to do partial ticks or things like that because they encourage you to just recast the spell. (Source)