Agreed. No way I'd ever downgrade from SQ F to SQ J or LH F to LH J.

Remember that LX is another option which will be a longer journey with more connections but very nice (and better than LH in many ways). If flying FRA-ZRH-BKK-SIN then you get FCT at FRA, the FCL at ZRH, very high quality seats on the long sector and also excellent food/service. Plus, you can do cash & miles on LX F.