Its quite possible to transmit old style Teletext in digital broadcasts and it is done in many other countries. It is because of the choice of features in the standards picked for the UK that this is not available. Many digital TVs would in fact receive them if they were broadcast and the country setting for the TV was set to Germany rather than UK.
Having both services present (at least on a single channel) would get quite complicated and need some rules about how different key presses should be handled.
To be honest the time of teletext is now ending (as much due to the Internet, mobiles and tablets) but I still regard it as the only seriously successful information service provided on the TV ever launched. The Red button MHEG services main value seems to be access to hidden channels and I also see iPlayer and similar services that are coming to TV screens these days as just methods to access content rather than interactive/information services although they use those mechanisms. People don't really want to interact with the TV (apart from immersive games) but for a long period Teletext was the only source of realtime information in the home. Broadband replaces this need for most people and tablet/touch devices are a more convienient interaction method I don't predict that there will such mass use of TV based information services ever again.
On the main topic of the switchover I'm slightly surprised that it hasn't been accelerated as the progress seems fairly good and the percentage of houses now relying on analogue broadcasts is dwindling even where they are still available..