Development of a new airplane takes a long time. Trying to certify a plane by similarity usually takes 4-5 years, clean sheet design would take even longer.

Dassault's Falcon 6X (largely derived off the 5X that was cancelled due to design issues from the engine supplier Safran) was only just announced and isn't scheduled to go on sale until 2022, and a good portion of its design work has already been completed.