Marck, I don't think that locking the overheads while the engines are running is realistic, but I don't think your objection that people would just put their valuables in a bag under the seat would actually defeat the purpose.
All of us have gotten off a crowded plane before. Unless you're next to the door in use, you need to wait for the folks in the aisle to get out of the way before you can leave your row (at least in economy). How long does it really take to grab whatever you've stuffed under your seat? How much does, say, a purse or shoulder bag slow you down as you walk down the aisle? Is it really an obstruction in going down the slide? I expect that bringing this sort of bag causes almost no delay. If it hasn't already done so, the FAA should run some tests and find out.
As many here have recognized, instructing passengers (yes, as a paying customer, I actually do find the acronym "SLF" offensive, thanks for asking) to leave their bags behind doesn't work. It's sort of like the war on drugs that way. Screaming about how much you hate the people who do it won't work either. So, perhaps it would be a better ides to set a realistic goal (say, no wheelies or other large bags) and setting egress requirements for new aircraft that ensure a reasonable evacuation time even if not everyone getting off is a fit young person wearing nothing more than running shoes and a track suit.
Also, airline practices of making carry-on bags free and charging for every checked bag are just stupid, whether you look at them from the point of view of safety, comfort, quick aircraft turnaround or maximizing airline revenue. If anything, a bag over a certain minimal purse/briefcase size that fits under the seat, should cost more than the same bag in the hold to encourage passengers to do the thing that makes it easier, more comfortable and sometimes safer for the airline to get everyone to their destination. Creating financial incentives for passengers to make travel less convenient, efficient and safe is stupid and probably shouldn't even be legal.
As for me, in an evacuation, I'll probably leave my stuff behind (as a guy with my phone, money and wallet in my pockets, that's pretty easy), but if the risk seems low, the crew seems calm and everyone else is doing it, I don't know. I might bring the small bag under the seat with my laptop, etc. I would never bring a bag out of the overhead compartment though.