Probably because Taiwan has struggled to man their fleet of Vipers as a result of the end of conscription?
I think it's fairly objective to say that Taiwan continues to fly F-16 sorties. All reports say they've been scrambling more in 2020 than past years due to PLAAF activity.
My citing of the U-2 shootdowns is to illustrate the possibility of cutting edge American technology ending up in the hands of China
Yes, but you need to remember that by nature of the plane the U-2 was vulnerable in principle to being shot down because its role was to fly over enemy territory. That meant if it got shot down the wreck would almost certainly be captured.
In contrast the ROCAF doesn't perform flyovers of Chinese territory anymore, and even in event of a war with China I think the ROCAF would keep some distance from the coastline to avoid giving Chinese air-defences an easy way of bringing them down.
Not that I'm warmongering
No problem, I hadn't thought that anyway.
Who knows, maybe reunification can be achieved diplomatically within the coming decade, and China could get access to Fat Amy without violence.
I think the chances of that are so remote they wouldn't factor into US thinking. Due to Chinese aggression towards Taiwan I think the chances of the KMT winning the 2024 general elections are not high right now, and the chances of getting both the presidency and a super-majority in the legislative are even more remote. Even if they somehow did, unification couldn't happen without a referendum to allow constitutional change.
All of that by 2030 given China's current policies regarding interacting with Taiwan is about as likely as Xi Jinping turning into Zhao Ziyang and instituting internal Party democracy whilst standing down voluntarily.
Would America really risk the possibility of proprietary technology ending up in their adversary's hands?
If the US defends Taiwan from a Chinese invasion then it will have similar risks of losing F-35s to China. But as I noted above, I expect that would be mitigated by simply not flying over Chinese territory - not least because neither the US nor Taiwan would need to do that as they have standoff weapons.
Besides, I agree with you that F-35 sales to Taiwan are not going to happen right now. I would be surprised to see a sale before 2025 (assuming Trump doesn't get re-elected somehow).