I think they need to scrap the BBC Three and BBC Four brands and focus on BBC iPlayer as the master brand.
If programmes are streaming only, then they are "on BBC iPlayer".
It was telling when they launched RuPaul's Drag Race UK - one of the biggest shows for BBC Three - that it had to be advertised as "on BBC Three, on BBC iPlayer". Really, at this stage, that sort of mixed messaging is absolutely pointless.
If it's on TV, it's on BBC One or BBC Two. Otherwise, it's on BBC iPlayer. ESPECIALLY now that iPlayer is supposedly "personalising" recommendations.
The future of linear TV channels is premieres and big event television. There is no need in this day and age for the BBC to be spreading its content thinly across three linear channels rather than having two strong ones. Anything that doesn't need to be appointment-to-view can live on iPlayer, and anything else that needs streaming (eg Proms) can be on pop-up livestreams on iPlayer. This is the reality of television consumption today.