You really have a one-track mind.
'Just because they say so' is a reason you gladly accept in a lot of different circumstances. You're not going to need the passport to water your flowers.
Of course it can be there for the reason you mentioned too, there might be a point at which the vaccine passport serves as a 'no-covid-danger'-document. But it's not proven to do so, similarly to the possible side effects it can have.
I gave you the purpose I had in mind for what we can say for now, but you already know more than everybody else.
To add:
"You can be tested every time you cross a border. You cannot be vaccinated every time you cross a border," Thomas Crampton, chief marketing and communications officer for The Commons Project, told CNN Business.
https://edition.cnn.com/2020/12/27/tech/coronavirus-vaccine-passport-apps/index.html
Good on ya for swinging away from the masks.
You are intimately familiar with all other countries customs, holiday schedules, school schedules, festivities, national holidays etc, got it.