Unless your name is Jed Mercurio the days of 2005-2009 ratings are gone for television in general, let alone DW, so I'm not sure what metrics the Beeb will be using to judge their no doubt sizable investment in him and Bad Wolf on.
And it should be mentioned that Chibnall's series have still beaten 90% of the rest of drama on British TV in the ratings and aren't that far off what Moffat was getting for Capaldi's series.
Well unless you're Jed Mercurio or Call the Midwife, but yeah this is true, I think it's the drop off that concerned the BBC more. S11 had great numbers but even within S11 the drop off was more pronounced than with most seasons of modern Who (they pretty much all start high and then drop before reaching a plateau, but Chibnall Who didn't seem to even out, it kept dropping.) Then S12 came along and the trend continued. The AI numbers (for what they're worth) are also pretty poor for Chibnall's seasons.
It may be that ratings have levelled out now, and maybe today the best Who can hope for is around the 5 million mark, whether that's Moffat/Capaldi, Chibnall/Whittaker, or even RTD/?. It's depressing that Whittaker started on such a high. A lot of people showed up for her debut, and yes a lot of that was the novelty factor of a new Doctor (and a woman to boot) but clearly large numbers stuck around for the opening run of episodes. There was a new audience waiting to be grabbed but instead they were allowed to drift away.
I was talking to my goddaughters at the weekend, both of whom became Who fans because of Whittaker, especially the younger one, and while they still love Who, they haven't watched all of S12 yet! (it got a bit preachy as their mum remarked).
The world has changed. Who has way more competition than it did in 2005, maybe even more competition than since the Smith era so I hope the BBC have realistic expectations for RTD. My plucked out of thin air, no scientific thought whatsoever, gut feeling is that if RTD can consistently get the rating back above six and a half million then it'll be seen as a success, especially if this comes complete with increased critical approval.