If the politicians give them the finances to do so.
I've been involved on both sides of the fence
the attrition rate is part of the assessment matrix - it is factored in
govt decisions on final numbers aren't based on the exclusion of the attrition estimates. whatever final numbers come out of the final contract include the attrition estimates
if you get 4 then someone will have run the numbers and said that you will lose one frame in the next 30 years to either platform faults or human error.
sometimes the strategic and tactical decisions will get rewritten and other artifacts will be sought to try and back-fill if that fleet is determined to be critical to the future force structure and the immediate threats to the national interest.
RAAF is a good example when you look at changes in fleet numbers for F/A-18A/B F/A-18F, Growlers and F-111
In the examples of rotary assets - it was also impacted by changes in service force development and purple force developments. the critical fulcrum to the changes that affected other fleet (air, land and sea) elements was the LHA decision
eg there's a tendency for some to look at the LHA's purely in the ARG/ESG role - when a big unstated role which has far broader implications is the ESB role
at the ESB role, lift and changes to supporting rotary around that lift demand are front and centre. that resulted in broader and suddenly rapid turnarounds in heavy, medium and light lift fixed wing fleets getting finalised.
in the case of F-22 and the B2 closure of the lines is the prerogative of the govt of the day - the attrition numbers still stand whatever the cap becomes - just in a different ratio. the argument around whether those numbers can be defended by the govt when the services wanted more still doesn't alter the fact that there is a crunched up number which then re-adjusts the attrition figures
the other thing is that attrition out of assessment necessity is a platform centric value - govts when they make a decision expect the services to come back and work with the adjustments on the overall capability impact.
eg if "we" lose F-22 is that picked up by JSF and more effective air to air, if "we" lose B2 is that going to be picked up by forthcoming long range hypersonic strike
the value propositions for platform/fleet are not the same as capability/tactical/strategic intent/posture