That's the galling thing with "Britbox" for me. Primarily, it's taking shows away from other platforms we have/had, aside from Nerflix they're free platforms, and putting them behind a subscription paywall. Broadchurch used to be on Netflix and was one of the leading titles that they reported ITV were taking away from the service in order to put it on BritBox, before they'd even decided they were calling it that.
Some people may be happy/financially able to subscribe to all these different streaming services but me and my girlfriend already share Netflix and Amazon (and we only have that for the delivery options, the streaming platform is a bonus) and I have NJPW World for wrestling. Apple TV, BritBox, Disney+ etc. are all just not worth the additional outlay.
It will be interesting once this is more established what BBC and ITV do to balance their platforms - they have to think about what they're offering between BritBox, the iPlayer/Hub and their actual channels, since the secondary channels are often full of old shows being rerun as well.
For instance, I happened upon a repeat of Fleabag after the News last week and enjoyed it so much I want to watch the entire series. Right now I can do that on the iPlayer so I probably will watch it and, if there was ever another series, that's a new viewer for BBC. But if it was behind a paywall? I doubt I'd bother. And if they just didn't air a repeat because it's behind that paywall then I'd never have even had the opportunity to stumble upon it. So it's a tricky one.