I think, seriously, that the P-8A is a different case from the RJ. The P-8A base design is a civilian aircraft with a full FAA/CAA/EASA paper trail back to the year dot. The RJ base is a 1950s military aircraft, out of production for 50 years.
The P-8A design was a single, recent program; the RJ combines several major separate mods (mission equipment, re-engine, glass cockpit) done over many years, all a long time ago.
Leasing P-8A is a fast, low-risk way back into ASW/MPA, but one that has some high costs associated with it. It's disturbing that it seems to be being done without an analysis of alternatives, particularly one that addresses other ISR needs.