No, it is not implausible in the sense that "it could never happen". However, neither was it implausible for this to have happened with the myriad of other viruses which have jumped into our species across our natural history—and yet here we are.
Remember there is no motivation behind the virus to adapt, nor such thing as "wiping out" the immune response. At most, there are random mutations which, if they are more successful than the previous ones, will become more widespread than their alternatives.
Are mutations that are less sensitive to current populations' immune responses more likely to succeed? Yes, of course! But there is a practical limitation to how much a virus can accumulate mutations which evade immunity to previous variants: it must not break its ability to infect (human) cells. Eventually an equilibrium is reached, as it has thousands of times previously.
Remember there is no motivation behind the virus to adapt, nor such thing as "wiping out" the immune response. At most, there are random mutations which, if they are more successful than the previous ones, will become more widespread than their alternatives.
Are mutations that are less sensitive to current populations' immune responses more likely to succeed? Yes, of course! But there is a practical limitation to how much a virus can accumulate mutations which evade immunity to previous variants: it must not break its ability to infect (human) cells. Eventually an equilibrium is reached, as it has thousands of times previously.