Dissecting TDSR (Total Debt Servicing Ratio) introduced by MAS (Monetary Authority of Singapore)

It has recently been reported in the news that there are many people whom have been unable to refinance because of the Total Debt Servicing Ratio (TDSR) that has been introduced by the Monetary Authority of Singapore (MAS).  Usually, policy implemented should not be retrospective, i.e. should not apply to people who already bought and financed their properties.  However, TDSR has affected many people retrospectively, when they needed to refinance to get out of the higher rates being charged by their banks after the lock-in period (which usually incurs much lower rates during the lock-in period). 

Not sure whether it is due to TDSR, or may be due to a combination of other property cooling measures like ABSD (reducing number of buyers significantly), the mortgagee sales (banks' forced-sold properties) advertised seem to have more than doubled after the introduction of TDSR. 

Because of the above, my curiosity was arose as to how TDSR is being computed and why suddenly so many people are affected?  So I decided to take a deep look into the TDSR components and dissect it, and have made the following discovery in the process:

1) Your free-holding S$ cash has depreciated current value in the eye of MAS, because MAS instructions to banks is to only considers 70% of your free-holding S$ cash value as the computable asset value into its TDSR calculation....

2) Somehow, free-holding foreign cash is so much more inferior to S$ cash in the eye of MAS that MAS instructions to banks is to only considers 30% of your free-holding foreign cash value  as the computable asset value into its TDSR calculation (as compared to 70% of value for S$ cash!)...

3) Even if you pledge your foreign cash to the banks,  MAS instructions to banks is to only considers 70% of your free-holding foreign cash value into its TDSR calculation (vs 100% for S$ cash).....

4) Your other free-holding financial assets (like shares, bonds, gold, unit trust holdings etc) are only worth 30% of their value according to MAS instructions to banks in calculation of TDSR.......

5) Your other financial assets (like shares, bonds, gold, unit trust holdings etc), even after you pledged to the banks for their loans, are only worth 70% of their value according to MAS instructions to banks in calculation of TDSR.......................................

I am really bewildered about all the above though, is MAS expecting foreign cash to depreciate by 40% against S$ to have such instructions to banks to consider its value at only 30% of current value vs 70% of S$ current value?  Not only is the above 40% depreciation assumption totally unrealistic (because currency won't depreciate by so much in short-term), BUT I would have expected that going forward, it is very very likely, >90% chance, that S$ will depreciate against other major currencies like US$ and hence it should be S$ cash that should have lower value than Foreign cash instead? 

Is MAS expecting other financial assets (like shares, bonds, gold, unit trust holdings etc) to depreciate >70% to give banks instructions to only consider 30% of their current market value in calculation of TDSR?  This is at a time when global economy is recovering?  Won't it be more realistic to expect further gains of such financial assets, and hence they are of more value than S$ cash on hand??? 

All the above rules for computing TDSR not only seem arbitrary to me, but also do not gel with reality, such as why free-holding foreign cash is only worth 30% of its market value while free-holding S$ cash is worth 70% of its market value in the computation of TDSR etc!!!!........................