It seems like a bet that global markets will continue to outperform Singaporean markets, and I don't think that's a sure thing.
Shiny, with due respect, I think you've got this one backwards on this occasion.
If you're going to overweight a particular country's stock market, that's actually a bet that that particular country's stock market is going to outperform the global average. VWRA and IWDA also include SGX-listed stocks. If DBS goes on a tear and spends the next 20 years soaring to global valuation heights, becoming the next (in valuation terms) Amazon, Apple, Microsoft, or even Facebook -- hey, I cannot
totally rule it out

-- then VWRA and IWDA will pick that up, too.
No, your basic argument for overweighting the Straits Times Index of 30 stocks that happen to be listed and traded on the Singapore Stock Exchange is that individuals who expect to retire in Singapore (and who have that legal right) ought to overweight Singapore dollar correlates because that's the currency they'll use to support their real lifestyle needs starting some decades in the future. That's why you're suggesting some stock de-diversification, which involves some greater stock portfolio risk. But it's a calculated additional risk in an effort to reduce another risk, currency risk -- to orient to the future retirement spending needs of particular long-term investors.
I agree with the general principle here -- it's sound. But I don't agree with that large an overweighting on a mere 30 stocks listed in one tiny, moribund stock market that soon. I think it's better to ride along with the global average stock market performance to a greater degree then slowly, progressively, ease into a more Singapore dollar-oriented posture within the 7 to 10 year period before retirement. I would also point out that the Singapore dollar itself isn't like most other currencies. Singapore is a small, open economy that doesn't grow wheat, doesn't have large beef herds, doesn't have natural gas reserves, and so forth. Tons of goods and even services are imported, and the Monetary Authority of Singapore (MAS) manages the Singapore dollar as a loose peg to a trade weighted basket of major currencies. There just isn't a lot of long-term currency risk in that, unless we think the MAS will somehow fail in their currency policies.
You've explained previously that you are not a fan of currency hedges in funds themselves. Neither am I. Well, overweighting SGX-listed stocks is a crude currency hedge. (It's crude because, for example, Singtel, one of the STI stocks, is a majority Australian dollar denominated business now.) Should we be advising investors to take 30+ years of currency hedging, on a currency that's loosely pegged to a trade weighted basket of other currencies? I don't think so. How about 7 or 10 years? How about some milder overweighting? How about not 50-50 SGX-Global for 30+ years?
Of course there's also the very real possibility that someone who expects to retire in Singapore doesn't actually end up retiring in Singapore, especially when that person is a 20-something or 30-something young investor just starting out, and as so many Singaporeans (and practically everybody else) fall in love and marry across borders.
Anyway, I agree with the principles, but I disagree with overweighting 30 stocks in one tiny stock market so heavily so early.
A 50-50 split, or thereabouts, actually makes some sense when it comes to U.S. investors. Stocks listed and traded in U.S. stock markets currently are hovering around 5X% (or even up around 60%) of global investable stock market capitalization. Before the recent introduction of truly global stock index funds, U.S. investors had to assemble their stock portfolio using at least two funds: a S&P 500 stock index fund (for example), and a global ex-U.S. stock index fund. And a 50-50 split works pretty well in that case for those investors. But they don't have to do that any more because now there are funds such as VT that do it all automatically. Also, the U.S. "textbooks" properly should be adjusted when translated to the Singapore context, and it just doesn't make sense to drag a 50-50 local-global split across a 30+ year investing life when it comes to the SGX and the Singapore dollar.
Anyway, if somebody wants to do something like this allocation:
20% MBH
15% ES3 or G3B
65% VWRA or IWDA
then save diligently/doggedly, then start adjusting that allocation starting 7 years (or as many as 10) before retirement to be more local bond heavy (certainly), that's fine! That's a perfectly sound approach, very reasonable in the circumstances within this market and this currency.