ok
let me give you some case study.
this is Aleppo, a city in northwestern Syria
size:190 km2
population: 1.8M
it took the SAA a brutal 4 years stalemate, in a house to house fighting, whilst having an army of (basically) angry civilians turned rebels.
en.wikipedia.org
the SAA had complete aerial supremacy over the rebels during the 4years long fight. an infinite amount of supply of artillery, men and machine. not to mention a relatively safe supply line into the city it didn't fell in 24 hours.
here case study#2
City: Mosul
Size: 180 km2
Population: 2M
en.wikipedia.org
it took the might of approximately 100.000 Iraqi men, the non-stop 24/7 US+Allies aerial and artillery bombardment, tons of supply and above all
an open supply line all the way from Baghdad to Mosul. it took the coalition 9 months+ to capture the entire city as well as it's surroundings.
now here's Taipei.
Size: 1,140 km2
population: 2,646,204
do you know that you only have like twice a year to do an amphibious landing in Taiwan?? otherwise the weather and current will not allow to do so.
not to mention you could only do that in some beachhead which is situated in the western flank of the island.
The Taiwanese would have ample time and preparations to detect and locate PLA massing point on the other side of the strait, the US will likely provide them with this. by that time they would have time to position it's missiles and long range artillery to basically turn your massing point into oblivion.
even if let's say you manage to miraculously have enough transport ship to transport your troops, you would still have to face losses from Taiwanese anti-ship missiles.
The Taiwan strait is like 180km wide, in peacetime you'll need hours to cross the strait, in wartime it'll took longer consider that you'd have to slow down to avoid minefields, etc.
if you manage to land what's left of your troops in Taiwan beaches, you'll have to fight Taiwanese men in those beaches, which not even in the best of situation will take a day.
after you secured the beachhead, you'll somewhat have to do with your limited men on the island to face and subdue the entirety of the island as well as it's cities, where ROC troops are waiting in ambush. Urban warfare and mountainous terrain as I have pointed above greatly benefited the defender.
the situation that Taiwanese military faces in regard to the PLA is not as dire of that the Syrian rebels and ISIS against the SAA and coalition.
24 hours to reclaim Taiwan is a fantasy.
that city (Taipei) is just one you would have to conquer, there's a lot of mega urban settlement in Taiwan, like Taichung