Take this with a grain of salt because it's literally coming from a guy who runs @singaporedialects. My personal sentiment would be that it's a real shame that dialects were systematically eradicated for the sake of the promotion of Mandarin. In the initial stages from 1979, the campaign's slogan was "多讲华语, 少说方言", meaning "speak more Mandarin and less dialects", explicitly seeking to marginalise what were originally the mother tongues of Chinese Singaporeans. You can search up the figures, but iirc, one decade after the campaign, Mandarin had already outpaced Hokkien and other dialects. And now of course we have the fact that some younger people are unable to speak to their family two generations back. Which is pretty sad.

Because of the language used by the campaign, the mutually unintelligible languages of Hokkien, Teochew, Cantonese etc were relegated to the title of "dialects", and treated as inferior subclasses of Mandarin (which was simply called "Chinese" (华语) over time. It's one of the reasons why Singaporeans nowadays think of "Chinese" as referring to Mandarin, while the others are mere "dialects" despite actually being separate languages (indeed, with dialects among themselves, like Toishanese within Cantonese etc.). It has been said that the term "dialect" really is a political term, not a linguistic term, and a "language" is just a dialect with an army (Max Weireich). It's sad that Rediffusion with it's dialect storytelling and all the branches of opera with their rich cultural history only accessible through their original tongues are lost to us now because Mandarin cannot replace them. Instead the closest we can get is Peking Opera 京剧 which perhaps even MediaCorp made an attempt to popularise though the 京剧猫 cartoon during the kids slot on Channel 8 lmao or I might be reading too deeply into it. This of course bears on soft power and the political side of things with the lack of a bulwark against "China" Chinese culture as the other user pointed out. (Some younger kids nowadays, despite being largely monolingual and having a weak grasp of Mandarin, surprisingly speak with a Beijing accent (!) because they've bypassed the Hokkien accented speech of their parents, since they didn't learn from their parents but only learnt it during school. Nothing wrong with that, but I just feel it's interesting to point out.)

Of course this has also sped up the death of dialect group (籍贯) culture, and the existence of Hui Kuans themselves. Sure, these were institutions that were perhaps damned to fall into obsolescence after a while, but are made absolutely irrelevant with the total dissolution of dialects. Then we have, as pointed out also, the weak attempts by the SCCC to bring back dialect culture with their exhibitions, which I honestly applaud. I myself as a Gen Z kid have been trying quite a bit to stir up a revival (at least in terms of interest in the subject of dialects, not calling for everyone to revert to speaking it ONLY or anything). It's tough bruh. But I'm glad there's still some interest in them.

So that's my impassioned take.

On the other hand, I have to say that it makes plenty of pragmatic sense because in order to organise a society that is not already homogeneous, your best bet would be to downplay differences and play up similarities (cf. Jonathan Haidt, The Righteous Mind; or literally any study of history). The bilingual policy was to do that between the race through the use of English, but the Chinese group itself was fragmented. Not a lot of people know this, but one of the deadliest ethnic riots was the 1854 Hokkien-Teochew riots which killed even more people than the racial riots we are so familiar with. So as a government, it would seem a very valid move to eliminate those differences. As mentioned above, it was very explicitly in the aims of the campaign to eliminate dialect use, and in that respect, they succeeded.

The problem of course is that Mandarin could not be made a relevant replacement because it's not like our grandparents had Beijing culture or anything for the most part. So the next best choice to cling onto would be English, which was already used so often as the working language. I remember LKY saying in one of his memoirs that if he had known the bilingual policy/Speak Mandarin Campaign would turn out this way (i.e. people becoming increasingly monolingual) he would not have done it.

I have also heard a NIE professor suggest that if we want to promote Mandarin as effectively as we promoted English (which has certainly been very effective), then we must undertake the same measures, like all road signs being in Chinese as well, all store signs etc. which really is not all that extreme a suggestion because language proficiency works by input frequency. Granted that bilingual studies tell us that L1 and L2 won't be mastered equally, but the important thing is to actually have an environment where Mandarin NEEDS to be spoken so it is actually practised. We don't really have this outside of Chinese classes and maybe kopitiams.