Proton Mail is operated by Proton AG, which is a for-profit corporation.
That being said, even though Proton Mail is probably more trustworthy than Google and Microsoft services, it's still handled by a for-profit corporation and therefore can't be fully trusted.
Nowadays if something is owned by a corp I wouldn't recommend anyone to get too attached to it. Use it while you feel it's worth, but prepare to swap for something else eventually.
In other words: don't ever fully trust your data to company owned software, and always look for a backup solution.
Microsoft is trying to "reinvent" Xbox every couple years, for the last 15 years. Not even themselves trust their own plans, since they never stick to it.