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.