Yes, it's a bad thing for D&D long term. Or at least has the potential to become so if it grows to become significant.
Why? Because competition is good, and exclusivity stifles competition. In this case with other digital (VTT) formats/platforms. WotC has a poor reputation for digital, they need competition in the digital space or their track record shows it will be bad for the community.
Finally, I wish folks would get away from thinking PDF is the only, or the undeniable best, digital format. Even an ePub or mobi format would be better than PDFs. And there are better formats than that.