The government has spent the past few years pushing various local and indigenous languages (MTR and Bus announcements in Taipei are made in 3 languages already).
Adding English as an official language doesn't seem likely to change anything.
Once again, there's a historical link in those two countries that isn't there in Taiwan. India is also a completely different ball game as there are political ramifications with all the different regional languages involved. In Taiwan, they are trying to make an artificial construct which is not needed and makes no sense.
To me it is a meaningless act. One of Hong Kong's official language is English, while in the US there is no official language. Is English more prevalent in Hong Kong than in the US? The Taiwanese should invest more effort and money in promoting English education. But in the end English proficiency won't help if Taiwan continues to be self-contained rather than having an economy that is open to the world.
Well, your suggestions don't seem to be doing much for a place like Korea. The country spends about $5 billion USD per year on English instruction. The country is "open to the world," yet proficiency rates are far below similar countries. In fact, Korea and Taiwan have a similar English Proficiency Index (EPI); 53 and 52, respectively.
In order to learn a language, there has to be a relevance, a desire, a good infrastructure and a way to interact with speakers of that language and ideally an immersion period. Asian languages are very different and it is just as hard for then to try to learn English as it is to an expat trying to learn Asian languages which many do not even though the opportunities are far better than those of Asian citizens in their own country. It takes a lot of work and time and it's understandable that most can't be bothered. Even in countries like Canada who claim to embrace bilingualism, only a small percentage of the population is fluent in the two official languages. Same goes for the US where few English speakers bother to learn Spanish.
That being said, I personally find the progress with the level of English in many Asian countries remarkable in the last 30 years.
The pervasiveness of the English language has spread due to the internet, social media and content providers, such as Netflix. The vocabulary of young people has eroded and they often resort to English "loan-words" to plug gaps in their mother tongue. So yes, English is slowly chipping away at culture and heritage. I can speak more than one language and thinking is very different when done in English vs my other language.
I see little sense in Taiwan taking this approach as I personally saw very limited English use and skills in Taipei and the Greater Taipei area. For example, Hong Kong's English proficiency is way ahead of Taiwan's though I don't rate Hong Kong's general English proficiency highly (both written and spoken). Japanese and Korean native speakers struggle with English more so than the Chinese primarily because the grammatical structure is vastly different to English. Chinese follows an almost identical grammatical structure, so it's a matter of learning the vocabulary and improving pronunciation by practice. Japanese and Koreans need to fundamentally shift how their minds think about language, which is a very difficult task that can't be fixed with improving vocabulary.
I almost feel that it is unfair to make English an official language. Yes it is a remarkable opportunity for business growth and global entrepreneurship and partnership - however I feel like it will take away from the present cultural identity.