The Taiwanese government has admitted that their older fighters (F-5s, IDFs and Mirage 2000s) are not likely to do well against the newer Chinese Su-30s and Su-27s. Taiwan ran combat simulations and found that they would lose 2.8 Mirage 2000s and 1.7 IDFs for every Su-30 shot down. The F-16 is considered a match for the Su-30, and Taiwan is trying to get more of them. China is trying to persuade the United States not to provide them.
The 60 elderly F-5s are no competition at all for the Su-30s, and were only considered useful against Chinese bombers or their MiG-21 clone (the F-7). The locally made IDF is, and this is no secret, a second rate F-16 replacement. The 56 French Mirage 2000-5 are expensive, and manpower intensive, to maintain.
The Taiwan Air Force is considering retiring the older, more difficult to maintain, aircraft, and put more resources into the more recent models. Taiwan wants to upgrade its 146 F-16A Block 20 fighters, which are some of the oldest F-16s still in service. Taiwan has on order 66 F-15C block 50/52 fighters, a sale which has been blocked by local politics, and Chinese protests, for years.