In peace time the F-16V (or the M2000 or the F-CK-1) can do the air policing role (and would be better at it than some kind of dedicated point air defence defender).
We're discussing the replacement for F-CK-1(maybe F-16V, too, but unlikely) .
F-16V is considered to be less than ideal
interceptor - it's time from standby to take off and to intercept point (key metrics) are worse than that of F-CK-1(per Taiwanese sources). It isn't unexpected really - F-20 was marketed to be noticeably better than F-16 in this regard, too, and even tiger II (!) was viewed higher.
For relatively small Taiwan with its relatively small warning times, this metric is crucial for wartime employment, too.
That certainly shouldn't be understood by vigilant users as "lol he went nuts, f-ck-1 more capable than f-16v/f-35".
This is a just two metrics, which are important for a particular country in particular geopolitical and geographical realities.
It doesn't challenge the fact that F-16V is obviously the most capable and advanced a/c in Taiwanese service, and F-35 is far beyond that in capability. Both are true. Just not in this particular aspect, which most of its users don't even really care for.
And in wartime mobile SAMs will have limitations but likely to do considerably better than airfields and aircraft shelters against Chinese missiles and air strikes.
Dispersed light fighter units can operate from straight patches of roadways, and aircraft can be hidden, serviced, fueled and rearmed literally under tree cover/within buildings, by skeleton ground teams(even using drafted and mobilized troops, when aircraft are designed for it). False targets and SAM/AAA network complicates finding them, too.
Again, just a few weeks ago we saw fighter units being based within
artillery range of the opposite force(also within LR SAM range, btw), and that force was neither lacking fires nor was it blind.
In a theoretical scenario where, say, those mobile SAMs have very limited survival then the PRCs application of these same capabilities against Taiwan airforce assets would similarly see them as equally short or even shorter lived.
Key disadvantage of SAMs is that they can maneuver only as rapidly as wheeled vehicles do - they can't proactively react to enemy AF incursions. In the end, concentration of forces wins.
Defender's fighters
can react and they
can intercept those incursions with concentrated effort. Moreover, that threat is of a completely different nature from SAMs, forcing the attacker to fight two completely vastly different opponents at once.
if some of the Taiwan airforce can survive for a prolonged period then it would be important that they have range/ endurance and weapon load to hit hard and to widen out area from which it could operating from (and which PRC would then have to search and target, reducing the chances of being picked off on the ground). A very light weight short range point defence air defender is not a good match for that, and point defence would still be better done with mobile SAMs.
Main strikers are F-16s, but even now F-CK-1s are anything but harmless.
New fighter will clearly be asked to do at least as much. We aren't in the 1980s anymore when it was something outstanding.
And whilst I would be sympathetic re: the potential non-supply of the F-35 there are wider aspects to consider like Taiwan needing (but potentially finding it hard to obtain) foreign assistance to build anything like an equivalent capability as the F-35.
No one asks Taiwan to make a 3-in-1 bleeding edge all-trick-pony. Good enough is the name of the game.
ROCAF clearly needs something that can at least try to reasonably challenge the ever-evolving opponent(which, accidentally, deploys well over a hundred 5th gen aircraft today), that isn't so directly tied to the whims of some people from half a globe away. As a bonus, such an aircraft can be tailor-designed for local requirements.
As an additional bonus, practice has shown that the US often changes its mind on not selling something to ROC as soon as it is ready to deploy a similar capability on its own. Thus, ironically, it may very well be their fastest path to Lightning, too.
So two capabilities for a price of one.