Thank you for the question.
I just want to clarify that, although I teach in the tax department, I'm an economist. I wouldn't want to speak for my tax colleagues.
However, it is widely recognized among economists as well that predictability is extremely important for investment planning, among other things. Announcing a measure without being able to provide the corresponding legislation is not an ideal way to proceed. I'm not stealing their words, but many of my tax colleagues and tax professors were a bit shocked by this approach. It's certainly not ideal for people to be able to plan their affairs properly.
As for the possibility of declaring this gain on a prospective basis, it seems to me that this avenue would also have made life easier for all those who, without necessarily saying their circumstances were difficult, would have wanted to rearrange their affairs to respond to a situation of greater neutrality. If they had had the opportunity to realize the gain before this measure came into effect, it would probably have made their lives easier.