I would not agree that the latter is cheaper.
Besides the renewal test costs being the same I also make allowance for the fact that if you allow the SPIR lapse, there is mandatory training requirements and that makes it very expensive to get it back in the future.
Keeping the MPIR and/or the IMCR valid does not get round that requirement.
My friend paid a lot of money to get their SPIR back even though they were current MPIR and current instrument flying. The Authority specified (as they can) a minimum of 4 hours MEP SPIR training...........a total waste of time and money but those were the requirements.
The CAA tried some years back to separate recreational and professional flying by saying that pilots who hold a CPL or ATPL would also have to get a PPL in order to have PPL privileges. It did not go down well then and I can't see much evidence of a shift in professional pilot opinion.
Regards,
DFC