Is Wuflu bound to evolve to become milder? No. That does not always happen. For example, smallpox remained highly lethal for thousands of years. Falciparum malaria remained dangerous over thousands of years, as did sleeping sickness.
Consider myxomatosis, introduced to Australia in an attempt to exterminate those wascally wabbits. Originally, it kill > 99% of infected rabbits. But after a while, rabbits were evolving greater resistance, and the virus changed as well. It only killed 75% of them, or even as low as 50% in some cases. Utterly harmless !
It has continued to evolve: it now has some capability to shut down the immune system, more effectively in lab rabbits than in wild rabbits, that have been co-evolving.
Is this coronavirus being selected for lower virulence? Understand first that it already has fairly low virulence – it only kills a few percent of those infected. It is already mild, as such things go.
Myxomatosis virulence was near 100% – it had nowhere to go but down. You could certainly imagine tradeoffs that reduced virulence and increased transmission. Not the same story for Wuflu: at most it could only gain a few percent of fitness by becoming milder, and if that entailed lower viral production and less transmission, becoming milder might well reduce viral fitness. Indeed, since it seems to be transmitted efficiently in hospitals (a significant percent of all Italian medicos are now infected), higher virulence might be favored.
Consider the American elm, or the American chestnut. How are they doing nowadays?