I just think it's unrealistic to think paid RE work is going to fill this need.

I think there are two realistic options: 1) the manufacturers suck it up and agree to support devices with timely updates over a longer lifespan, or 2) manufacturers open-source every bit of software that runs on the device.

#2 seems less likely, given that a lot of hardware is driven in part by loadable firmware these days. On the other hand, if that firmware is chipset-specific and not device-specific, and the chipset manufacturer can commit to releasing security updates for those, at least 3rd-party OS images could pull them in without help from the device manufacturer.

But really, it's all about demand: Apple tends to support hardware with new releases for 4-ish years as a matter of course, and i-device users are accustomed to expecting that. Android users just don't expect that, and your average user doesn't understand security enough to get why that's such a big problem. They likely mostly just think, "oh well, I won't get the new shiny Android version Jane has on her new phone, that's ok". If average users can be educated to the point where they will switch manufacturers if they're not getting security updates for the useful life of their phone, the manufacturers will listen to their declining sales. I just don't expect that to happen.