Certainly, the Roadster Mk II will be all-Tesla. But when they built the original Roadster, it was much easier to buy the rolling chassis and just concentrate on the drivetrain.
And in my not-so-humble opinion, they did not make a mistake in choosing the Lotus Elise for the body of the Roadster. It's an absolutely gorgeous car, and my biggest complaint about mine is the aftermarket Alpine "infotainment" system, which was an option I ended up with because I bought an existing car from inventory, and it was the least-optioned one available. I honestly don't think that at the time, when they were just starting, they could have built a better rolling chassis. Note that they barely had enough capital to bring the Roadster to market. If they'd had to do the R&D on the rolling chassis as well as the drivetrain they might not have been able to pull it off.
The next Tesla Roadster will be a far better car. But I don't think they could have built a better car than the one they did at the time. I'll have to sell mine when I get my Model 3, because I don't have space for another car, and I want the safety features mine does not have. But I'm really going to miss it.