Yeah, one day coal will be replaced with something else. That is how industries grow, they change. Electricity will not always be generated with coal, but that day when the last coal mine shuts down is a long way away. Coal is still a viable energy source. Essentially, because coal (and fossil fuels in general) are so cheap, it allows us to expend resources on other things whereas solar and other so called 'clean' energy sources take more resources to produce the same amount of energy. Prager University has a convincing video on the subject.
One point Clinton tried to make was that now is the time to shift from coal to 'clean energy'. Again, Prager University has a good video on this subject as well explaining why it is actually better for the environment and humanity in general to remain on coal and other fossil fuels.
The point is not that one day coal mining will be a thing of the past in the energy industry. The point is that Clinton has specifically called out coal miners as people she wants to lose their jobs. This is clearly not based on some rational thinking that now is the time to switch from coal to 'clean energy', these are private businesses and the market will determine when the appropriate time to switch is here. Clinton wants to put coal miners out of business because she views them as her enemy.
Every politician needs an enemy. The political left does not like coal because they buy into the global warming hype. Clinton is part of the left so she has to name these people, who are doing nothing more than living their normal lives, as her enemies. The families, the lives and the jobs are just collateral damage.