We don't have to worry about the future of Doctor Who. The BBC has said that they find Doctor Who such an important show that even when the viewing figures drop they will continue making it.
This season was pretty good, although not a huge improvement over the last. It remains uneven, with better and weaker episodes. It's good that they turned the humor down a little when the actors are just not that funny, but also that's what's lacking from how it was before: Tennant, Smith and Capaldi were just so funny.
It's great that the last episode goes deeply into the reason for regeneration and why the Doctor can regenerate beyond what is possible for Time Lords. I don't know all the history of Doctor Who (and some episodes were lost), but as far as I know that hadn't been done before.