J-10, J-11, J-15, J-16, H-6 have very high operating and maintenance costs. The too much use in Taiwan airspace will reduce its life and strength and can make them repeat the fate of F-16 falling.
If China wants to harass Taiwan, the effective solution is to use J-5 and J-7 aircraft, it's cheap, powerful and easy to produce mass
Of course, J-5 and J-7 are not opponents of F-16 and F-CK, but it's not a problem. Taiwan did not dare to shoot these planes, because it was a war declaration. China can to punish and attack them.
You have it the wrong way around.
Those flights serve valuable and desirable military roles, they are not done merely for the sake of harassment or geopolitical signaling.
Flying over water in international airspace at those distances from Chinese domestic airspace, sometimes in larger daily packages, are important for training and readiness and familiarity in operating over a region that could be a potential future area of conflict.
Given that, you would absolutely seek to fly aircraft in that airspace which would be involved in any such conflict in a regular way that would provide usefulness while not giving away useful intelligence, in this case for them being 4th generation fighters, bombers and various MPA, ELINT/SIGINT and AEWC aircraft.
In other words -- these are flight hours spent proactively with actual benefits for military readiness and training.
Flying old third generation fighters misses the point entirely.