It needs an external lens anyway so you could shove it into the camera assembly, waterproofing is not an issue.

Cost is one, but this isn't something you can measure quickly on demand, you want to keep averaging multiple measurements over time. So unless you want to hold out your phone for minutes to get a measurement it doesn't seem practical.

Where did you see 30 euros? If so that's quite good I think - I remember (a long time ago when I was researching air sensors) Honeywell's laser sensors were similarly priced. A low price point will absolutely help Bosch drive adoption.