This is an important point. Having two very distinct addresses (different lengths, different prefixes, different RPC APIs) makes it very obvious to users when they have the benefits of shielded transactions, and when they don't. Thus users make an explicit choice to forgo privacy when they use transparent addresses.
The problem of only 28% of transactions currently being shielded (https://explorer.zcha.in/statistics/network https://explorer.zcha.in/statistics/timeseries?hashrate=fals...) is a separate ecosystem problem, where usage of shielded addresses with third parties (like wallets and exchanges) requires them to use new APIs, instead of just interacting with the new block chain via the Bitcoin API. IIUC Monero has also encountered these issues, and it is something we are both working on improving.