The issue is we keep rebuilding the industry, get things ticking along just fine, then shut it all down and buy off shore for a generation, then rebuild it all again. We also conduct hideously expensive mid life upgrades and life extensions that deliver dubious capability gains at great expense. Overall we spend huge sums of money for no net gain in defence capability or economic benefit.
It has been known for years that it is more efficient to maintain a capability than to drop it a rebuild it over and over again for the sake of saving some short term, up front, expenditure during boom times when it is cheaper to buy overseas. It is cheaper to replace ships at twenty or twenty-five years, without a midlife upgrade, let alone a life extension, if you factor in the costs associated with having to rebuild an industrial capability that has atrophied while you were upgrading and life extending the old ships.
There is also the simple fact that an upgraded old ship will never be as capable as a comparable new ship and such a vessel will struggle to achieve fifteen additional years. It will also be more expensive and difficult to maintain, often achieving lower levels of availability and capability than a new build that would easily last twenty or twenty-five years.
Its a big picture thing, not just through life for a single platform or type but a holistic view of maintaining and adapting a capability through changing times. Short term savings made because of a budget crunch, or because of an extended period of overvalued currency making imports cheaper, will inevitably end up costing future generations more, either in outlays to rebuild what was lost, to continue buying off shore or in a reduction in capability, hence strategic options to protect national interests.