You're partially right, but without the BBC other broadcasters wouldn't even be attempting to compete. Downton Abbey only gets made because the BBC have made other comparable class type period shows. If the BBC weren't doing that we'd just have more populist cobblers to get eyeballs on adverts.
forgot to reply to this - I'm not buying that else you wouldn't see commercial broadcasters in other countries also make these shows. Fact is plenty of these dramas are commercially viable - ITV has had it's fair share of period dramas in the past - I remember watching Sharpe and Hornblower as a kid for example.