I guess there is always room for improvement. I don't worry about the SDM in the 16-50/50-135 since the hack was released that let you convert them to screw drive. The 16-50 could be better optically and it has been out for, what, seven or eight years? Wouldn't be a bad thing to see an update, both optically (and to stick a faster motor in it). Not sure I could afford it, though. I got my 16-50 for under 600 dollars new, back when you could get amazing deals from Prodigital 2000 in Canada.
In the end, the lenses are what keep me with Pentax and they are usable, even if people do bash them a lot.
(16-50 shot)