Jonathan,
I've been thinking about this question since it was suggested by Paul Theroff.
There are many possible motivations behind the creation of Earl FORFAR:
- Genuine gift from The Queen to her son
- Recognition of him being a "reliable" pair of hands in recent years. His is the only first marriage of her children to last 20 years+
- Desire to give him a Scottish title to match other Royals where some convention about use of a Scottish title in Scotland seemed to be forming. However, if Edinburgh was still the plan, he might only use the Forfar title for a short time.
- A backstop against Scottish independence related to the above. Whilst the Independence movement seemed to accept keeping the monarch as Head of State, at least in the immediate term, this does not mean they might not change certain things, such as giving permission for titles linked to Scottish titles. It seems to be settled that the UK government seeks permission from foreign/commonwealth powers if using their places in title TDs. This is the only possibility I can see that prevents the Dukedom of Edinburgh being re-created.
Remember in all of this that there is the Duke of Edinburgh Awards Scheme, probably Prince Philip's greatest wider legacy. The Earl of Wessex is a trustee and the clear plan seems to be to make him Patron in due course. It would all be very tidy if he is the Duke of Edinburgh for these purposes!