Not sure why "inevitable" unless you mean that its growing dominance is 'inevitable' given the randomly created happenstance of more contagious.
The virus is constantly replicating, with each instance a chance of a minor, inconsequential mutation. But every once in a while, the mutation is consequential.
And those less contagious don't grow and infect as quickly as those which are, so are sort of crowded out by the faster spreading mutation.
Likewise, the random mutations of lethality, more or less, are a factor of the number of replications, no particular direction is inevitable.
But it's really, really bad news when the mutation proves to be more lethal.
So, as there's more and more spread, there's more and more replication, more and more chances of a lethal direction mutation.
And, if people can get it and spread it, even having been infected previously, then there's an ongoing high chance of a bad direction mutation.
So, lots of infection is bad news...if there ever happens to be a bad mutation on lethality.
Though obviously it would be great if the lethality happens to get better, whether through mutation or because vaccinations and therapies are so effective and easy.
So, we're looking for two things from vaccines, 1) that vaccination/boosters etc reduces the negative impact and 2) that it reduces contagiousness, or better yet stops altogether.
It's already clear that we need boosters to achieve either of these things with reliability. And #2 remains a bit iffy, though certainly appears better.
And it's also clear that prior infections are less protective than adding vaccinations.