This seems to ignore the existing, wider audience that enjoyed the movies.

I won't defend the quality of the Rings of Power—I think it's lacklustre at best—but the premiere did get 25 million people watching it. It seems doubtful a completely original series would have attracted so large an audience right off the bat.

And I don't think there's necessarily a problem deviating from the source material when adapting something. But you do need to make sure your new ideas are as good, or better, than the original. To me, the single biggest failure of Rings of Power is that those deviations have been so poorly executed. Bad plotting and bad writing abounds.