Since GSM is better used for voice call than WCDMA (coverage, simplicity) and LTE provides highest data rates, how can WCDMA be still used in future ?

In denser clutter areas like in cities. WCDMA service gets better penetrated through obstacles than LTE and at the same time WCDMA can give better throughput than GSM. This reduced penetration loss makes WCDMA still stand in future.

LTE along with CSFB gives the best monetary benefit for now for an operator. There is VoLTE commercially deployed as well but I still don't see any monetary benefit for an operator in that.

WCDMA is3G technology and service providers have invested in the infrastructure, the usage will continue for reasonable time in future. However, keeping in view the more complicated architecture and infrastructure requirement for and costs thereof WCDMA as compared to LTE,  it is predicted that service providers as well as the equipment manufacturers will prefer to go for LTE application in the coming years.Effective and efficient use of spectrum also advocates the usage of LTE in contrast to WCDMA.      

3G must and will remain in future. It provides better capacity and less cost.

In Japan, for example, they use only3G and LTE, GSM is canceled there.