There were no $12B laying around on their CapEx plan.
With all my knowledge of the industry, building a fab can not be an overnight decision in any way. Fab expansions, economic analysis can go for years before first blueprints are drafted.
If TSMC had anything like a second 5nm fab in mind, it would've at least tipped off investors at confidential meetings, and it wouldn't have been such a bombshell as it is now, and the biggest funds invested in TSMC wouldn't be scrambling to write "TSMCs matter being currently investigated" in their own IR press releases.
With how it is now, seeing a very expensive 5nm fab with just 20k starts per month materialise without any place in the strategic vision, makes little sense.
For anything else, just building a bigger 5nm plant, or extending it later would've made way more sense.