Why did Hashem say "v'Rega Adaber"?
Malbim: Sometimes Hashem says to build only for a moment [if the nation sins] - like the first parable, in which the Kli was broken and nullified.
Did He only speak about building and planting?
Radak: He actually did so, and then they did evil, and He reconsidered. I needed to explain so, to distinguish these two Midos, for evil will not take effect if they repent, but good takes effect in any case, since Hashem said so. However, it will not last if they do evil. This is like Yirmeyah told Chananyah ben Azur (28:9. If a Navi predicts Shalom, when it comes, it is known that he is a Navi Emes [and if there is war, he is a Navi Sheker].) There is a hint to this here - refer to 18:10:1:1.
