I see most people buying DVD because of price, and of course, because the option is still there. Bluray has been around for almost 15 years now, and DVD is still here - DVD should’ve been discontinued a decade ago. If Bluray was the only option on the shelf, they’d buy that instead. Old formats need to go in order for new formats to make any impression on the sales stats, and to make it relevant.
If when Bluray was released, a DVD copy was included in all Bluray releases, the discontinuation of the stand alone DVD release would have made way for a genuine Bluray take over - we’d now be seeing Bluray at the top of the format sales figures, and things would look very different, no matter how you view the stats.
The reason DVD is such a massive success is because it isn't 'videotape'. Being able to buy a film on a CD sized disc encouraged millions of people who rented VHS films to buy them instead. If DVDs were removed from the market 15 years ago, the result would have been the vast bulk of people who bought DVDs stopping buying discs altogether.