Evidence for the idea that more people would aspire to platinum if platinum benefits were better? Well, basic human nature for a start, plus the experience of people like myself (I'm currently hovering just below platinum but don't feel super-motivated to actually go for it).

You're right, VA's competition is QF, and QF has major concrete benefits at the platinum level (access to snazzy F lounges worldwide... plus a few things we get at the gold level on VA such as priority security). For many people the choice is between VA platinum or QF platinum or VA gold + QF gold, and VA isn't doing much to make VA platinum the preferred option.

One idea for VA platinum benefits that don't cost anything (or much): limited vouchers for anytime lounge access. It's annoying when I see someone off at the airport and we can't get into any lounges. The old "anytime lounge access" perk was too easily abused, but make it a limited number of times a year and you've got a genuine benefit.