No I get it, but that is another issue for another day.
First thing first. Capital gains is only for an appreciation of the flat price. If you buy a flat for $350,000 and sell it for $200,000, where is the gain? Also don't forget that by the end of 99 years, your flat value is zero.
You raise several issues here. Capital gains happens only when the flat you bought can be sold for more money. In recent times, this has not always been guaranteed due to the high prices for new flats. Earning Capital gains is a function of timing. For example, if you "bought" a 4 room flat for $150,000 in the 80's, and sold it today, you will have capital gains, its not an issue of if you will get it, but an issue of how much capital gains you can get today. If you bought a new flat today for $500,000, and sold it 3 years from now, the odds of getting capital gains of any significant amount is greatly reduced. The ability to earn capital gains is therefore a function of when you bought it, and when you sell it.
Going back to your question of a capital loss, this situation to date has not been faced by many, not such a big loss of $150,000 as in your example. Mainly due to the much lower prices paid in the earlier generations. You did not mention what is the time line in your example. But you have to consider the following factors:
1) Let say that you held the flat for 20 years and you sold it for a loss of $150,000 as in your example above. Don't forget that there is a theoretical monthly housing savings component when you sign a prepaid 99 year lease with the HDB. Lets say that when you "bought" your $350,000 flat, the cost of ownership per month (mortgage payments, conservancy fees, property tax, etc.) was $1800. No an unrealistic number. The alternative to buying this unit would have been to rent a similar one. How much would this exact flat rent for? Could you have paid a rent less than $1800 a month? Not likely. The rent would probably be more. The spread between the rent and what you have to pay monthly for housing is an actual benefit. If you save $250 a month on average over the 20 years, using a discount rate of 2%, you would have saved $29,000 by "buying" this flat, thereby reducing your capital loss. In actual fact, you will be held to the whims of the landlord and have to renew every year, and I am sure the rent will increase. So, the savings could be much greater, But you see, there is an actual savings here.
2) Also consider what will happen to your CPF if you had left it in your account without taking it out to buy a flat. If 20 years ago, you did not withdraw say $100,000 to buy your $350,000 flat. In some years, CPF paid you as low as 1%, now its 2.5%. But the real rate of inflation was much higher. In some years, we had double digit inflation. Therefore, you would have suffered an actual purchasing power loss equal to the rate of inflation minus the rate you received from CPF. This spread is in my estimation about 4% a year. If your $100,000 suffered a 4% loss over 20 years, it will be worth only around $70,000 in real purchasing power after that. If you take it all out to use on a flat "purchase", you have a fighting chance to make some capital gains versus a guaranteed loss if you left it in CPF. This is the reason why the PAP sets the CPF rate so low. Even though they take your CPF and make double digit returns, they pay you only 2.5% so as to encourage you to pull it out and use it to buy their over valued flats. If CPF paid everyone 10% for their money, who would want to take it out to buy flats? People will leave the money in their CPF and try and use other funds for down payment. This would defeat the govt. goal of bleeding the CPF.