It doesn't mean you "have too" but it does mean the actual number of aircraft we can Introduce in comparison to other nations at the same time is less. So for example where the US and USSR for example have multiple examples of aircraft we could introduce across a certain period, Britian had just the Phantom over the same period. There are also 10 nations now. Not just a single one.
Other aircraft like the Harrier GR.7 have been introduced and it's currently one of the best aircraft in Air RB. It may not be personally fun for you. However, we cannot add what simply hasnt been there all this time. The Phantom was the only choice for a Rank VII "Fighter" if you want to classify it as such. The Harrier however is a more than capable aircraft too. Whilst it may not be to your personal taste, again there's not much we can do about that as Britian doesn't have much in the way of airframe variety from this point forward when it comes to top rank jets.