We all knew (hopefully) that would never happen (keep their promise). It's nothing China specific. Even lots of elected politicians fail on it in any country.
I think hindsight might've clouded your assessment of the attitude during the time pre-HK crackdown.
It was originally a time of optimistic hope that China would become more and more democratic, and the integration of HK back to the mainland will not have been hard (at least, china would meet somewhere in the middle).
There was already a wave of people considering, planning and did actually leave pre-1997 and around that time. A few did go back to HK afterwards when things looked better but left again due to the current issues.
I don't know who was optimistic. The West? Definitely not those in HK. It was a time when anti-China sentiment was high and supported by the media, artists, etc as the trend (vs the crackdown now). There were songs and campaigns supporting those that wanted freedom. Learnt from history many of the older generation that have witnessed the war and after knew of how CCP and army worked.
No 1 wanted a death sentence either. Those have worked in the mainland know the "law" doesn't exist as it does in the West (including HK at the time). Gang activity vastly dropped post-1997 because of a fear of the death sentence. A lot of them and their money left for other countries.
British HK was an act of aggression and a national humiliation for China.
The ROC/Taiwan was also happy to see the British go and their regret was that the communists succeeded where they had failed (and in fact did nothing at all as all foreign settlements were kicked out by the PRC).
Why would ROC/Taiwan be happy about it? It helps distract the PRC from ROC/Taiwan. The focus has shifted to Taiwan recently because HK has been "dealt with".
British HK was an humiliation for China. That does not mean for the PRC, that does mean China as a whole and a whole people. Both sides for the straight were glad to see the British go.
It's a lack of cultural and historical understanding not to realise that.
Likewise the focus since 1949 has always been Taiwan. It's HK that has been used as a distraction and 'tool' by the West.
