Could coronavirus variants become so lethal that 80 to 90% of humanity dies from them during the next 10 to 20 years. Yes.

 If you study pandemics and plagues you see that some especially plagues killed everyone except 5% to 10% in some areas of the world who stayed near where people were dying en masse.

So, it's theoretically possible that because we have 8 billion potential hosts worldwide that more and more lethal variants to easily arise in remote areas of the world and then become the dominant variants around the world and mayby 80% to 90% of humanity might die before vaccines can be developed soon enough to save enough people to stop this.

This is theoretically possible. The reason you aren't hearing about this is because they don't want to panic the public worldwide about this very real possibility.

The reason this is possible is because EVEN IF you vaccinate 80% of the people in North America and Europe this does NOTHING for the rest of the 3rd world places where the virus continues mutating for the next 3 or 4 years or more while almost no one there can get a vaccine.

And mutations from ANY part of the world have a good chance of eventually reaching all parts of the world, even here in the U.S. especially in people who haven't been vaccinated which likely will be at least 15% of the population of the U.S.

So, even if everyone else  (80% of the U.S.) is vaccinated against the original virus it may or may not have any effect at all upon any or all of the variants.