the reason why people are so concern about hdb prices is because they are sensitive to it...
open market shares are determined by sellers as well as buyers... you think that buyers will still go around happily buying to continuously form the 80% market share?... if yes, you are already making the assumption.
and all you are relying is your restrictions.. with the checking of new PRs/FTs, new local buyers almost all taking the queue to buy from hdb (to save 6 digits figure), where do you find enough buyers to make up the 80% market shares?
when sellers outweigh buyers in the open market, what do you get?
If Ramseth made an assumption, it's a reasonable one.
For the sake of argument, let's say GMS won and managed to get his "Delink" policy through and new flats are going for 100k per unit. You need to remember that there is a limit as to how many new flats are available for sale. If a buyer (for whatever reason) needs a house to stay and he either missed out on the ballot to get the new flat or he does not qualify (e.g. second timer), he has no choice but to purchase from the open market.
Yes, as suggested by you, he can sit and wait. Also, he will not be "happily buying to continuously form the 80% market share". But because of the limited number of new flats available, the restrictions on new flat ownership, or even his bad luck at missing out on new flat ballots, he eventually has to buy a resale flat. The exception is if he has enough money to buy a private property but in this case, it's irrelevant to the argument because prices of new and resale HDB flats will not concern him. His income will also disqualify him from purchasing HDB.
Will GMS's policy cause disequilibrium? Possible but unlikely. Other than the lower prices of new flats in the above scenario, his policy is simply a continuation of the situation now with the new flat restrictions, limited numbers, etc. New flat buyers will still be a subset of all HDB flat buyers but the latter may not qualify to be part of the former. Like the situation now, this group of people will always need to buy resale.
This is of course only true if we assume (yes another assumption) that the HBD does not flood the market with new flats, remove all new flat ownership restrictions, tear down resale flats, impose new restrictions on resale flats, etc.