Giving back HK might have been the only sensible move back then, and it might have bought HKers time and avoided a more open conflict, that wouldn't have ended well for HK.
At least Wikipedia disagrees with your sentiment, that HK was never part of China. Well, technically you said "PRC", maybe even intentionally, and you could take some weird position of claiming, that nothing inside China is part of China, because it was a different entity before PRC. But then so do many countries all over the world lose any claim to their territory. Germany, after second world war, France after French revolution, most prominently the US, after its founding ... Historically, HK was a grab of land by the UK. Granted, they built something nice up there, but only after the despicable acts they committed historically in the region. If we get into what the UK did historically in the region, it will not lead to a moral high ground.
I am genuinely lost in your argument. You start against colonialism then justify Hong-Kong being reintegrated to China because they would have taken it by force anyway which is pretty much the same thing as colonialism.
You then pivot to arguing HK was always going to be part of China for a reason I find unclear. Hong-Kong was never part of the PRC before the handover so I don't really see the appeal to continuity.
Have you considered that people are not arguing for colonialism but actually against any form of coercitive control?