I'm not really sure what to tell you but all of this is pretty consistent with how Delta treats upgrades for their entire elite program. Medallion members are entitled to companion upgrades based on the priority of the lower of the two passegners, and if their companion has no status, they are upgraded at a "companion" priority that is below all medallions (it is not clear where that priority stands relative to SPG Crossover upgrades).

Since SPG Platinums are not entitled to companion upgrades, the lower of the two of your priorities is "no upgrade" and so you don't get upgraded.

If you don't have status with any other airline, it still seems like getting free checked bags and other benefits through Crossover Rewards is superior to having nothing -- and since, at the SPG Platinum upgrade priority, you would be getting upgrades very, very rarely, if at all -- it seems like abandoning Delta may be a bit of overkill. That said, you may decide it is not worth it to you to fly Delta if another airline is cheaper, but occasionally taking advantage of the checked bag benefit through Crossover Rewards is worth something.

I do sympathize with your frustrations with Delta's call center -- unfortunately even the "premium" call center I reach as a Diamond Medallion is often no better than "acceptable" (though there are some great agents out there), and since Delta tweaks their frequent flyer rules constantly, even the best Diamond line agents often have outdated or inaccurate information about programs like this. Since you are calling the no-status call center, I can't imagine how bad they are.