I too have an original Black & Decker WM400 workmate bought in the 1970's, possibly from House of Holland, remember them?
It has rectangular tubular steel legs, something like 15 ply 20mm thick plywood tops, cast aluminium handles and steel elsewhere. It has been used, abused, sat on, stood on, hammered on sawed on, welded on, you name it, it's been done. So far, I've had to replace & repair some of the pivots, including new star washers, add sticky tape around the bottom of the legs to make a tighter fit for the feet, and repair one of the handles. I've also repainted it. I've never had a problem with it.
In short, as an original genuine article, it has been well worth every penny it cost, and is more or less just as sturdy today as it was when I first bought it.
I think, as people have said, the problem with the modern equivalents is that MDF, chipboard, and even pine boards, simply are not as strong as properly made plywood hence the breakages. (I've included pine boards because whilst they may be as strong in one direction, they definitely are not in the other direction.)
I have to say that I have looked at some of the modern equivalents, especially when I repainted & overhauled mine, and was not impressed.
Regards,
Peter G. Shaw