mmm Mr Biggus,
you ask should we "... keep something we already have, which works, or reinvest in something we have lost ...?"
Well if the something we have, (Sentinel) allegedly 'works' producing only wide area radar coverage over land - but for which there may be limited future requirement for (or political appetite for).
Then reinventing something we clearly lack but need makes a lot of sense to me.
Skint are we? There seems no end of money being squandered around the country. I feel it is a balancing act of available funds that will be needed to provide the necessary insurance we need to secure our maritime environment.