ADS-B Out is already mandated by the FAA starting in 2020, and many other regions will surely follow. (Although in the US not all ADS-B will be using the 1090MHz band, since the more GA / privacy friendly 978 MHz UAT will also be available).
Even without a mandate, many if not most commercial ops will want to opt-into extended ADS-B service because better tracking yields operational benefits and ultimately saves them money.