And by then one would assume that with only Typhoon and F35 in service, RAF eyes will be on more fast toys than in patrolling the borders.
You're wrong there - even in the fast-jet centric bits of the RAF that I am familiar with, there is unanimous opinion that loss of MPA was the single biggest loss during SDSR and the most obvious capability gap now facing our armed forces.
That's not to say that MRA4 could have been saved by cutting something else instead; there were other issues at work, as endlessly discussed above. However I think that future reinstatement of MPA would come significantly above further fast jet purchases or enhancements in future spending plans.