There seems to be a lot of assuming happening.
I'm not sure ditchable weight is ever used in technical diving. As evidence for this claim, you'll notice that technical diving is done with Backplate+wings or sidemount rigs, while ditchable weight pockets are generally only found on jacket-style recreational BCD's.
You're confusing ditchable weight for weight that is instantly droppable. In the days of bigger canister lights these were considered "ditchable weight". Stage cylinders are ditchable weight. Lots of things are both "ditchable" and "weight". In the UK there is bizarrely a preference for Kent Tooling reels which are solid stainless and weigh a ton and divers have indeed been forced to ditch these in an emergency to make themselves more buoyant.
I was also trained with a lift bag in AN+DP for redundant buoyancy, though it seemed to me less practical to whip out, inflate, and control in an emergency.
Again, it comes down to how you define things. In my opinion, a recreational diver with half a dozen dives having a loss of buoyancy on a wall dive might be an emergency. A technical diver on a wreck having a loss of buoyancy is an inconvenience.
It takes time to deploy something like a liftbag and not **** it up but you really should not be at this level of diving and let time pressure be a huge issue. If seconds matter in solving a problem like this then chances are you didn't take enough gas or you need to practice the skill more. If deploying a bag is part of your planned contingencies then make sure you have the time to do it.
I suspect the vast majority of younger open circuit techies nowadays are using back-mounted steel hp100's. My tech instructor was my age, mid-30's. He used those AND a steel backplate. Even with his drysuit he must have been very negative, which might be why he didn't consider his drysuit to be adequate emergency buoyancy.
When I used to dive OC, I used to use Faber 12's which don't look wildly different from the specs of HP100's. Maybe a kg or 2 less negative when full. With a pair of 12's, canister light, the weight of regulators and tank valves, all the other shite you need to carry... I was absolutely not "very negative". In fact, I needed a little bit of lead to be comfortable at deco. I've used everything from twin 7 litre/300 bar to twin 20 litre/232 bar and never found any of them to be unmanageably heavy in the water. In fact, the 20's trimmed out beautifully. I'd much rather use them than the 12's.
I'm not really sure what age has to do with anything though. I was 32 when I swapped to CCR so I don't know how a mid-30's open-circuit diver would feel about it. They may be more negative, dragged down by the sadness of not swapping to a rebreather before a mortgage, meaningless relationship and kids killed their dreams and chances for a fulfilling life.