There is a practical issue of giving users the option to send transactions transparently or privately. If majority of transactions are conducted without privacy, then most users are actually not benefited from such technology, and the minority of private transactions automatically become suspicious.
I strongly believe that privacy has to be enforced on every transaction to be most effective.
Correct. This is why no "privacy" currency with optional privacy will ever be successful at preserving users' privacy. This is true of Zcash, Dash, and Shadowcash. The only coin to get this aspect right so far is Monero.