What SSB tries to do is to achieve parity with 10-year SGS. If you want to liquidate earlier, that's NOT the right instrument for you.
Slight nitpick here. SSBs are potentially
great if you want to liquidate early, because of the embedded put option. If rates have gone up, you can sell them back at par, when an equivalent SGS's price would be much lower. (If you own pre-2022 SSBs, it might be worth comparing the yields against what you can get from a fresh 10-year SGS.)
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(1) What is the relevance of WA YTM for investors’ decision? The higher the better?
Not really, because a higher YTM almost always comes with some sort of tradeoff - whether that's duration risk, credit risk, reinvestment risk, whatever.
The YTM is "if you held the bonds in this portfolio to maturity, this is the yield you'd get. Some of it comes from coupons, some of it comes from 'pull-to-par' (the price of the bond itself moving toward 100)". More coming up just below...
(2) Does it mean that since current Yield is 3.19% p.a. whilst the WA YTM is higher at 4.21% p.a., there is a potential for Yield to move towards 4.xx% p.a. in future, assuming current interest rates ( which are used in current valuation of long dated bonds) remain more or less constant?
Nope. Because interest rates have gone up, a lot of the bonds in the portfolio are trading at a discount to their face value - maybe 85 or 90 cents on the dollar. (This is completely normal; it doesn't reflect a change in credit risk, just a change in interest rates.)
The YTM of a bond comes partly from the coupon payments and partly from the price drifting toward 100 cents on the dollar. The yield should increase a little over time as older lower-coupon bonds mature and are replaced by newer, higher-coupon bonds; but a decent chunk of the performance of the portfolio will come from those bonds drifting back from 80-90 cents on the dollar to 100 cents on the dollar, over a period of years.
The "yield" is just the coupons; the YTM is the coupons plus the pull-to-par.