Taiwan needs to purchase new aircrfat rather than sinking money into airframes with a lot of hours on them.
I would disagree the planes in Taiwan's air force are all purchased in the middle and the end of the 1990's and some are even delivered in the 2000/2001 period. That means the airframes in the Taiwanese air force are less than 20 years old. Those airframes still have 10 to 15 years left in them and are on average 10 years younger than most airframes in NATO Europe or even in the US. And Taiwan can choose to "zero hour" those airframes that would stretch the life of these airframes even longer. The age of those airframes is not the problem. The problem with Taiwan's air fleet is the technology on these planes. Things like all glass cockpits, helmet mounted sights, RCS reducing features on the airframe that are now common on the new PLAAF planes still don't exist on Taiwanese planes. So in that sense the upgrade package from the US is a good deal for Taiwan. Too bad that it only starts around 2016 and end in 2021.
Aside of the technology the other problems are the numbers and the platforms that Taiwan has. Taiwan in the air is aside from being outnumbered are also being outgunned and out sized by the Chinese air force. The Taiwanese are simply how should i put it... being "out flanked" by the Chinese.
I think a major hurdle is, who is willing to sell new aircraft to Taiwan without risking a bumpy relation with China, especially an economic one given China is now a credible alternate engine for the world's economy? No doubt the French would be happy to sell but not when it risks her economic relations with China. Would the US be willing to include Taiwan into the F-35 program? Doubtful. The US could stand the heat but that would make the "pivot" become a full blown Cold War II. Russia usually sells to just about anyone but not sure how receptive that deal would be to both sides, Russia is an on/off ally to China and Taiwan to the US.
It would seem the only viable alternate would be to develop an IDF-II but can she pour in the R&D dollars needed
The political, diplomatic and economic reality of the world today is that many nations these days literally cannot afford the ire of China. So there won't be any arms deals for Taiwan coming out of Europe, Russia or elsewhere except for the US. Even there the Americans has to weigh in the potential fallout with China in the kinds of weapons the US supplies to Taiwan.
As for a IDF II the problem there is that Taiwan did develop and produce a fighter in the 1980's and 90's called the IDF Ching-kuo. The project was succesful and those planes are still flying in the Taiwanese air force today. However in 1992 the US sold to Taiwan 150 F-16 block 20 and France sold to Taiwan 60 Mirage 2000 and the decision was then taken on Taiwan that after the initial production run of 130 planes that the plants and the project would be shut down. And after the last Ching-kuo rolled off of the assembly line in december of 1999 the Taiwanese locked up the plant and throw away the keys.
Most of the talents that the Taiwanese nurtured in the 1980's and 90's left Taiwan and most of them ended up in South Korea. You only have to look at South Korea's T-50 Golden Eagle and you will see the influence of the IDF Ching-kuo in it. Today Taiwan is looking for international partners to help them build trainer jets for its air force. Taiwans ability to build a IDF II is very slim indeed. And you also have to consider that a IDF II must have the ability to best planes like the Chengdu J-10B, Shenyang J-16 if not the Chengdu J-20 which makes it even less likely that Taiwan's domestic aerospace industry can produce a plane that could counter and upset China's numerical and technical superiority.
The problem for Taiwan in the China Taiwan balance is that Taiwan is not only competing against China's weapons inventory but also the science, technology and industrial base in China that have build them. And that becomes a losing game for Taiwan no matter what kind of weapons they can buy.
