Not at all. There is no reason why there can't be multiple versions of the Doctor on tv at the same time. In fact, it's better, because they can cater to different sections of the fandom. We could have a Romana show, a Ten show, an Eight show, a River show... The sky's the limit. Imagine if they were all connected in some way, with each being shown in succession until Fifteen has his series at the end of the year...