Yeah, I don't know if it's accurate to say we don't produce 'pure locks' anymore. It's not that long ago that we had a pretty enviable roster of locks and we have a few in the wings who look like they could be interesting prospects (Chessum Jnr, Kpoku*, Sodeke etc.).
The fact that Chessum Snr. and Martin are versatile enough to be effective in the back row is a good thing. It doesn't mean they're not locks first and foremost.
Moreover, apart from SA who does? If you think about France's locks over the past few years, Flament and Taofifenua are the only ones who were born and raised in France and Flament definitely isn't a 'pure lock'. Willemse and Le Roux were straight off the Bok production line and Meafou is just a freak. Ireland typically only have a few at a time, hence why they imported Roux, Kleyn, Treadwell etc. Similar for Wales and Scotland (albeit to a lesser extent). The Aussies have the Arnold twins, but the others aren't exceptionally big (Swain, Phillip etc.).
*As long as he doesn't elect to play for France.