Investors/traders can choose not to declare their capital gains if they trade with non-regulated brokers like IB.
Yes, they
could, and that would be illegal tax evasion. So is failure to pay GST legally owed (via private trade barter arrangements for example), paying employees under the table to avoid income tax and CPF contributions, etc., etc.
How is the authority going to check on that?
Oh, there are plenty of ways, starting with the fact that Singapore and the United States have a standing agreement involving the exchange of data for cross-border tax compliance. Interactive Brokers is required to send lots of data to the U.S. Financial Crimes Enforcement Network (FinCEN), among others. And it does. The Singapore government can get much of that data upon request, and it routinely does. I'm not describing any secrets here. These governments publicly acknowledge that they share such data.
Just to pick another example, tax agencies are also increasingly using social media analytics to figure out who's probably cheating on their taxes. If, for example, your spouse posts about the lavish family safari in Africa to Facebook, that might be quite interesting to IRAS.
Tax agencies also randomly select individuals (and households) to audit.
I'm barely scratching the surface here.
The U.S. collects approximately 85% of tax legally owed through voluntary compliance: Americans just paying their taxes as they should. That's
without caning and Changi Prison, two "special" tools the Singapore government has in its arsenal. I'm highly confident IRAS can and will collect the vast bulk of capital gains tax if charged with doing so, just as it collects the vast bulk of existing taxes.
Actually the capital gain tax on SGX stocks will just make investing/trading less attractive. The same reason if a higher broker or exchange fees are imposed.
To repeat, a hypothetical capital gains tax in Singapore wouldn't apply solely to SGX-listed stock appreciation. It'd presumably apply to all capital gains. SGX-listed stocks aren't currently famous for their capital gains, so a hypothetical capital gains tax might actually
help SGX-listed stocks.
However, that said, it's not a great argument even if you think a capital gains tax would somehow disadvantage the SGX. The government already effectively heavily favors and subsidizes SGX-listed securities through the CPF Investment Scheme, Supplementary Retirement Scheme, GIC, and Temasek, as notable examples. All that SGX boosterism is still not enough to keep the SGX from descending into global irrelevance. How much more government (i.e. taxpayer) support should there be to try to prop up the SGX?