[chimprich]

You don't need everyone to have it installed. Just getting a large proportion of people to use it would likely have quite a significant effect in reducing infection spread.

One compromise in making it semi-mandatory could be to reduce lockdown requirements for people using and carrying the app, because they'd be less dangerous.

[Freak_NL]

That would be discriminatory, and run afoul of many constitutions and treaties. It could really only happen if anyone could get the device and required data bundle subsidized. “Less dangerous” is not enough to warrant such a huge breach in citizen's rights.