If we're talking about one actor diminishing the quality of a movie, Lynda and probably any actress they actualy get for the role is a better actress than Gal, so if anything their interaction would have the weak link on the protagonist anyway.
Plus, the general audience usually don't care about good acting. Bad acting (and must be really awful) has an impact, but masses don't care about good acting in a summer blockbuster movie as long as it is serviceable. And Lynda's acting is GOOD. You just admittedly haven't seen anything she did and assume things. Being in a movie would actually mean being better directed than in TV.
I've said before, I personally don't care (but would be a bit disappointed), but not having Lynda in the movie (in any form) could actually hurt the success of the movie. It's stupid and seems exaggeration, but I know the WW fanbase very well. And I know we care more about the quality, but the movie has to succeed too. There are already enough stupid complains everywhere, and everything that happens to WW is always divisive.
Think about MoS... there were a lot of comparisons to the Donner films, even though there have been a lot more other Superman adaptations than Wonder Woman adaptations. There are already a lot of complaints about her being an warrior, the costume not being colorful enough, etc etc...
There isn't many of those complaints about Superman in the comics, but there already are a lot about Wonder Woman in the comics, by people who actually have seen the character evolve over time. So I'd imagine the movie version will generate even more controversy.
Most of the complaints about MoS are ridiculous in my opinion and I assume most of you agree, but it doesn't change the fact that it affected the box office. And remember that even then Snyder actually paid homage to Reeve.
At the very least she could voice Athena, if they don't show some gods and just have their voices (like in the animated film). I don't see how could anyone have a problem with that.