And guess what one of China's main focuses was? To take it back. Now that is theirs, do you think they would allow someone to take it? Do you think China would allow Taiwan to work against their interests?
You realize that US was incredibly close to nuke Cuba? Lucky for both US and Cuba, Cuba is an island so blockade was possible.
Now imagine Cuba having land border and is positioned on coast between DC and NYC.
China regained Hong Kong through negotiation and by the terms of the original treaty. They already "allow" Taiwan to work against some (not all) of their interests.
Russia has land borders with several NATO states, so I don't see how the Cuba comparison is relevant.
Every country in Russia’s position would do the same, or have already. You already mentioned Cuba, for China it would be Hong Kong.
In case of US, it would be a too long list of attacks of sovereignty whether through war or subversion.