I saw Tim Taylor speak at a conference a few years ago, he told how TT gets it's 1.7m viewers or whatever, week in, week out, with no publicity from Ch4 (I certainly almost missed the new series starting).
BUT, it has changed over the years, just try watching some of the very old episodes on UKTVRepeats. These days the reconstuctions look shoddy and quaint and they struggled much more to make a show when, frankly, they hadn't found anything.
I've actually enjoyed the presenter/co-presenter dynamic of this new series. Mary Ann is much more likely to listen than Tony was for starters and, as had been mentioned, she isn't just some 'airhead' but at least has relevant academic qualifications. Don't get me wrong, Mick will be a huge loss, but the show has survived a number of his prolonged, illness and (IIRC) gammy leg related, absences. Francis Pryor is much less annoying that he was when he first started to appear on TT (and has ALMOST shut up about the 'realm of the dead' and chucking swords in streams). So, if Mick wants to grump off let him. Not many shows can get away with peddling exactly the same format for 20 odd years and, whether the progression is slow or sudden they must change. Imagine if, I don't know, Blue Peter was the same as it was in the 60s...

Now, if they can just get a bit more Bettany Hughes and bring Dr Alice Roberts back I'll be truly satisfied...