The B737-800 is a run out model before the Max replaces it so switching wouldn’t make much sense. There are also A320 NEOs on order. The SilkAir B737s have a business class cabin and could be used on certain routes which might support a Scootbiz product similar to the B787 rather than the SIA full business class with lie flat beds.
Both aircraft types are well supported throughout the region and having a foot in both camps gives a stronger bargaining position when shopping for fleet replacement.
The company is now large enough to operate two similar types from different manufacturers, SIA have the A350 and B787 in service. Pilot progression would be easier for a new joiner starting on the B737 and moving through to the B787 as a first officer and then back to the B737 as Captain