Tombstone - The RAF has kept the Harrier because it has nothing to replace it and there isn't anything readily available . The Jaguar hasn't ever offered the load carrying capability of the Harrier in 'hot and high' environments.
It's also worth pointing out that the Jaguar is a late 1960's design which reached the end of it's development potential. The Harrier II is at least a late 1970's design and has had more scope for upgrades. As for the validity of the
STOVL concept - it's a nice thing but in reality it opens up problems with complexity and a greater chance of engine damage which can be critical.
Whilst the opportunities for overseas 'airfields' might be deminishing - reality is that effective denial of airfields is something of the past and I cannot foresee a possibility where a conventional aircraft couldn't operate
from some form of runway or indeed road successfully.