Why not try some totally new and revolutionary? Have the option of selling the HDB back to govt on a sell back scheme like those old flats, but only now its much newer flats for certain amount like 80% of market value and govt will provide a fixed monthly payout equivalent to the current rental market so in return you secure a rental yield over the next 10 years or something, subjected to negotiation to ensure that the rental yield will definitely be more than the 20% of the flat which the sellers forfeited.
This will make many HDB owners surrender their HDB flats rather than holding on to it for rental yield over next 20 years or something! Its a take it or leave it scenario. If you sell to govt, you take some earnings to buy your next property and still have rental income. This deal would only hold for the first 10 years after MOP.
Then HDB will do up these flats like DBSS and sell at a slightly higher price with some added features like alarm and security for each individual unit, etc (can brainstorm further). So can tackle several problems at one go.
1. Can ensure that HDB resale supply is not terminated.
2. Can have continual supply in matured estates instead of looking for new land to build HDB in these places.
3. Help HDB upgarders fulfill their dreams of staying in condos.
Much to debate about but that's just my whacky suggestion!