Factually, your timeline is not accurate. SAR 21 started coming into SAF service around 2000. And even then, in small quantities trialed with units like Commandoes, etc. The SR 88 was already available in 1988 and the even more improved SR 88A was available from 1990. Instead of switching production from M16S1 over to the SR 88 in 1988, and then stop production in 1999 or 2000 when the SAR 21 took over, they continued to make the M16. Why? I don't know other then to assume it was politics and PAP incompetence. When they decided to continued to manufacture the M16S1 under license for that period of 12 years, they were paying royalties thru the nose to Colt. and even worse, they could not export the M16S1 for foreign sales because the US govt. had the last word as to who they could sell it to, and Colt would not have permitted it.
They could have saved a lot of money making the SR 88 and also have a decent chance at export sales. Their stated goal to develop the SAR 80 was to get a cheaper weapon and get out under the yoke of Colt and the US Govt. They succeeded in doing this, yet they did not use the weapon for themselves. That is really strange, unless someone at MINDEF or CIS or PAP or Familee got a nice kickback to kill the SR 88. happens all the time in the US.
Now, is the SR 88 better then the M16? I would guess not, I have not used or fired the SR 88 myself. But i am very familiar with the SAR 80 and M16. If the SR 88 is in between these 2 weapons in quality, then I will say its an acceptable rifle for the SAF. I mean, are we intending to go to war any time back then? Would we have required a weapon superior to the M16? The answer is no. U are right when you say its a trade off. The SAR 80 was markedly heavier then the M16, but the SR 88 is lighter then the SAR 80, but still one lb heavier then the M16. But weight is not the only consideration. After all the AK-47 is a lot heavier then the M16, but it does not make it any lesser of a weapon. SO, the locally made, cheaper SR 88 with good overseas sales prospects is not made the standard rifle of the SAF and instead the expensive M16 is continued to be manufactured? where is the logic in that?
BTW, I am telling you here that the SAR 21 bullpup may be a newer technology, but it certainly is not significantly superior, if at all to a M4/M16, and hence can't be that much better then an SR 88. The SAF could have bought 300,000 SR 88 with all the money spend developing the SAR 21.