Another film that had a huge impact was monty python and the holy grail which was produced with a budget of 75p and ends abruptly because the money ran out. But they just turned that into a running joke throughout the whole movie.
It is a literal cop-out.
The regained interest surely allowed to assign more money to the series. DW was at the peak during 11th and 12th Doctors tenure with BBC America involvement in production - the low-quality is mostly gone and more CGI was utilized, and so the stories were good. Not mention the good chemistry between all main actors.
I suspect there’s a strong negative correlation between audience appreciation and production budget.
If you know you have 2 weeks and 100k to shoot an episode, and that everyone from the top down knows more money and time isn't coming, then you have no choice but to deliver it, no matter the corners cut. It's very hard to keep that attitude and pressure if people are aware that the budget limits are a polite fiction if you're convincing enough. (One might draw parallels to Steve Jobs' remarks about how the most important thing is not knowing when to say yes, but knowing when to say no.)
One might look at Team Cherry (Hollow Knight, Silksong) for an example of maintaining this - they made an enormous amount of money off Hollow Knight's success, and instead of scaling out the team markedly for Silksong, they mostly used the money as padding against any pressure to deliver it sooner, and worked at their own pace, to their own satisfaction.
But yeah, since 2023 Disney has been co-producing. It certainly explains some of the choices, but I don't know if they can be entirely blamed for the decline of the franchise.
BBC should back then pause everything and let fans take a break but instead they decided to continue with new companion. When that didn't go as expected they tried with new showrunner and Doctor; Moffat knew how to run this playground, Chibnall despite earlier good contributions did not. Then pandemics happen which resulted in delays - they tried with shorter series a'la classic show format of story split into parts but it was already too late. Not mention the stinking bomb Chibnall decided to launch to get few minutes of cheap thrills, nullifying over 50 years of creative contributions to this franchise.
Then there are socio-political elements but that just a minuscule of the whole problem.
For a casual viewer like me — I haven’t seen any of the new episodes — I’m struggling to find what this is referring to.
What happened with Clara and the doctors, and Capaldi, and what was this thing that nullified 50 years of the show?
The nullified thing is known as the “timeless child” storyline. The Doctor is retconned to not be a Gallifreyan at all and also the source of everyone’s regenerative powers (or something, I honestly forget). It was a totally unnecessary retcon but IMO much worse it was incredibly self indulgent. Most of the audience cares very little about the Doctor’s backstory, they just want enough setup to enjoy the adventures.