Firstly, Mark 16:15-16 doesn’t say that.
What I am talking about is when governments make the preaching of the Gospel illegal, or otherwise interfere with it via state controlled media and pressure on the churches.
Furthermore, neither Matthew 28:19, nor Mark 16:15-16, which merely restates what Matthew said regarding evangelism*, preclude a government converted unto Christ from acting in furtherance of this evangelization. It was indeed in this way that Armenia, Georgia and Ethiopia were converted to Christianity in the fourth century (except for a minority of Ethiopians who remained Jewish, and later, the Ethiopian state would include Muslims, and likewise in gratitude for helping to save Armenians from the Ottoman Empire’s genocide against them, the newly established Armenian state following the armistice at the end of the first World War (that armistice being the origin of Memorial Day and Remembrance Day in the US and UK on Monday the 11th of November), people of the Yazidi religion, which is a syncretic faith related to Christianity, were allowed to settle in Armenia, where they remain the largest religious minority (since it turns out that, most of the time, converting a country to Christianity makes the population more tolerant of other religions, the exception being Spain and Portugal following the Reconquista). Likewise, it was also in this manner that many other nations became Christian, including both the Ukrainians and the Russians, and several other related ethnic groups, and several other groups who were in turn evangelized by Russian and Ukrainian missionaries, such as the Siberians and many Native Alaskans such as the Aleut people.
So the conversion of countries beginning with their leadership is clearly a means by which the Gospel has been spread, although Christianity cannot be imposed by force, and I am not making that claim. Indeed, the Mexica people would never have converted from the Mesoamerican human-sacrifice religion had it not been for the beautiful icon, Our Lady of Guadalupe, painted by Juan Diego following the appearance of our glorious lady Theotokos and ever Virgin Mary - this holy icon caused the Aztecs to go from a religion based around human sacrifice, which was compromised by the Spanish conquest, but which the Spaniards had failed to displace as the popular religion of the people, even in those cities which were subordinate to the Aztecs and the victims of the Flower Wars, to worshipping God the Father, Son and Holy Ghost, and being baptized in Christ en masse and attending the Eucharist with a sincere devotion which remains even today despite the pressures of the secular world.
*Thus I am curious why you quoted the longer ending of Mark, which is of disputed authenticity, as opposed to using the more succinct verse and well-known verse in Matthew, which also as an added plus features the Trinitarian formula which is the agreed upon standard for baptism among all Christian churches I am aware of.