Keeping old website on one top-level, while putting new on different top-level

Hi there!

​

I/we have been building a new website/shop at our company - my boss is now suggesting (for the fifth time), that we launch the new one on one top-level domain while keeping the old one on a different top-level domain.

​

I have told him that this is a terrible idea, but since he keeps suggesting it, it seems he does not agree.

​

Can any of you help with some arguments for why not to do it?