Lkar,
You make an excellent and logical argument.
I also wouldn't normally buy a coach ticket and use miles to upgrade, but SQ is currently selling S-coded coach tickets for $1,860 that allow me to make over 30,000 Continental EQMs (at 150% EQM credit). It's a decent mileage run (2.4 cents per eqm) in a year where I expect to struggle to re-qualify for 1K. The upgrade to biz will cost me 170,000 SQ miles with the 15% online discount.
If I wasn't concerned with requalifying for 1K then I would completely agree.