Destroying nearly-new trucks for emissions reasons is a pretty questionable use of funding (and, yes, a 12 year old truck is still quite new) - and you're not going to be able to get away with a token few thousand dollars to encourage people to scrap them.
A decently maintained heavy road engine (tractor trailer) is a million+ mile motor, easily. A medium truck engine (think your typical toolbox work trucks, tow trucks, International boom trucks, etc) will do 300k-500k miles, and depending on how much the truck is used, that may be 20-30 years of operation. Same for the light diesels - they tend to have a practical service life of decades. My 24 year old truck is starting to be a little bit more rare on the roads out here, but I still see plenty...
If you know what to listen for, you can identify a lot of diesels by sound - and the International T444E (mid-90s design, the Ford 7.3 Powerstroke is that engine with a few tweaks) has a very distinctive snap at idle from the single shot injectors. There are still an awful lot of those on the road, and the youngest of them is almost 20 years old.
"Destroying 30-50% of the diesel fleet on the road" is not something I'd be particularly excited about - especially since new vehicle production isn't particularly environmentally friendly either. If you're specifically focused on the PM emissions, there may be ways to retrofit those older engines (at the cost of likely a substantial increase in fuel burn from the backpressure), but if you're going to hold them to the newer NOx standards, there's just no way to do it. They don't have the injection pressure and EGR systems in place to do it.
As of right now, they'd just be replaced with new diesels, because there are no electrics meaningfully on the market that solve the problems a large diesel engine solves right now. Plenty have been announced, very few are actually shipping, and of those announced, everyone is silent on their towing capabilities (I don't care if you can tow 15k lbs on a receiver mount, that kind of trailer weight should be on a gooseneck or 5th wheel hitch, and everyone is really, really silent on how their announced electric trucks fit either of those).
I also very much dislike "Cash for Clunkers" type programs in that they're one of the most nastily regressive programs one can possibly create. That program ruined the bottom end of the used car market for most of a decade, and permanently destroyed a lot of vehicles of a particularly easy to maintain and cheap to operate era (low pressure single point throttle body injection, not a ton of luxury features). It was a nice little handout to the next couple tiers up, but if you were operating in the "$100 car" realm (which were a thing at the time, I've owned 4 sub-$400 cars in the 2000-2010 era), it was absolutely devastating to your ability to find cheap cars. That sort of effective floor on vehicle prices for a while, followed by the hollowing out of anything below that price in the market... eh. Let's not do that again.
As far as coal rollers, though, the best thing that could happen is that everyone stops getting worked up about them and ignore them. They do it for the attention, and I'll suggest that it works really, really well. If you see one, get the plate, call it into your local emissions enforcement hotline if that's a thing, and move on with life. Everyone getting all wound up about them on the internet is exactly what I expect a lot of them enjoy about it anymore.