Can there be a loud piano?
I say yes, and that we should use it more often.
Soft dynamics, sung quietly, can be quite fragile and apt to struggle. But in my estimation, the biggest factors determining the impression of dynamics are timbre, tone, and diction – not objective volume in decibels.
So practice singing a loud piano. And then a quiet forte. Observe the dynamics while singing at volumes conducive to effective singing and musicality.