In an ideal world yes it would make sense to use it rather than build a new airport. Snag is the customer airlines won’t like it at that location. They need it on the outskirts of London pref to the W/NW. That way it can benefit from shorter travelling times to other destinations in the UK. Quick links to London, HS2, M1, M40, M4 etc
To use Manston you need to connect to HS1. Even then that will not suit a lot of people who might decide to drive.
If however the decision is to move away from London and put in better infrastructure then I’d agree it would make more sense to develop Manston than Boris Island.