Why should one party to an agreement hold the other party accountable to the terms of the agreement?
If you were mugged and then the perp finally left you alone if you swore to do his bidding for another fifty years, you'd also tell them what they want to hear I assume. But I doubt you would honor your agreement once they no longer had power over you.
Or germany that tried to invade not so long ago?
Or russia that killed parts of our royal family?
Times change, and the point about the treaty was guaranteeing the rights of the people that lived in the place now. In the same way that signing an extradition treaty with germany is about allowing citizens to enforce rights. (rather than an exploitative regime using it to punish political dissidents had it been in place in 1936)
the treaty meant that instead of having to evacuate all the hongkongers (only a small amount were "truly native") leading to a huge economic collapse and negating the entire point of getting the land back (and the island that was annexed through good old fashioned cannon)