If the exemption is pulled forward to say next year, then 100% of properties sold using this scheme next year wil be removed from the rental market.
Does that not assume that:- (a) 100% of these properties will be sold; and (b) 100% of these properties will be sold to owner occupiers?
Neither assumption seems very realistic to me.
TBH, I really don't see how the expiry of the 7-year minimum holding period could have any significant impact on the rental or sales market. In any event, I think there is zero chance of the 7-year period being extended.