There is a cost in renting the transmission bandwidth for the red button video services, a cost in providing the services with video (even if most of the video does originate from existing material - an obvious exception being Springwatch Unsprung) and managing its broadcast, and a cost in curating the Red Button teletext-type services. Take your pick.
It seems to me we are better able to do without the teletext services than the video feeds - nothing on the teletext service can't be readily obtained by an Internet search from other sources.
It is just the Red Button services and NOT the Red Button+ Services.
"Exploring a phased exit from the broadcast Red Button service and focusing our interactive TV offer on connected televisions and iPlayer"
The word 'broadcast' gives it away and emphasised by "focusing our interactive TV offer on connected televisions ".
One year we have Red Button. A few years later we have Red Button+ but no Red Button. Hopefully the BBC have not just balanced the budget but also have evaluated that Red Button+ is on the whole worth the budget but Red Button isn't.