Like everyone else I love Joanna. But after the calibre of the last two doctors I just don't think her acting would cut it, frankly. And we just can't have a woman. Sad but true. Otherwise we'll get caught up in the sexist debate every time she strays back anywhere before about 1980 and that will become the story, detracting from the main story.
I think Lumley would have proved herself in this role myself. I also think the sexist debate would have made for some interesting episodes and perhaps taken a different slant on history, even making younger viewers aware of past inequality. Dr Who does love to moralise. I think it would've been good, but as I said, it was only a rumour.