ITV - I think it went downhill after Thames lost its franchise and Granada and Carlton took over many ITV franchises, although I think the downhill spiral stopped around 1997, the downhill spiral returned with Central having its Central cakes removed and News at Ten getting the axe the first time round, I think the downhill spiral stopped once again around 2001 when News at Ten returned, but it went downhill once again with the arrival of the Celebrities idents in 2002, the abandoning of regional identites, the ITV Plc merger and the axing of News at Ten again in 2004, though it got better with Downton Abbey.
CBeebies - I think it went downhill starting in 2009, when it started to remove reruns of many good shows (like Brum, Pingu and Fireman Sam).
BBC One - I think it went downhill after Doctor Who got canceled in 1989, but got better, until the downhill spiral returned when the globe was axed, it got better again around late-2006, but the downhill spiral returned again in 2017 with the Oneness idents.
I strongly disagree with you regarding ITV and BBC1. Sorry.
DW was already in decline come '89 - as, indeed, was TOTP. On the other hand, EastEnders was doing very well indeed, while Noel Edmonds still had loads to offer on Saturday evenings.
And changes in ident don't necessarily mean instant or near-instant changes in quality, do they? BBC1 already had problems when Oneness arrived.
As for ITV, well, some will say that it's been *all* downhill since Thames lost their franchise - and I'd strongly disagree with that too. After all, there *was* that big-money quiz show hosted by the guy from Tiswas...
And even in the five-and-a-bit years between Thames coming off air and that show arriving, there was Inspector Morse and A Touch of Frost, as well as Heartbeat before it got stuck in 1969. The Bill was still a proper police drama, too, and of course Corrie and the 'Dale weren't on *quite* as frequently as they are now. And World in Action was still going.
Axing News at Ten the first time was undoubtedly a bad move; ditching the regional identities in England and Wales and introducing the celeb idents weren't particularly good things either. But, again, things like these don't necessarily mean instant or near-instant changes in the overall quality of the channel, do they?