"Solid, hard-yards user research that comes from a large enough sample set"
Unfortunately, not so simple in my experience. That's a lot of time and money many companies can't or will not fork over. So, shortcuts and approximations are used as an alternative.
Agree, unfortunately that is the tricky aspect:
evaluating the cost/benefit of undertaking user research (with personas as one aspect, along with diary studies, interviews, observational studies et al - depending upon the nature of the tasks/problem to solve.
What definitely costs companies a huge amount of time and money is launching new products or concepts into markets - especially without some form of pre-qualification i.e. is there a need? does this solve a problem? will customers actually pay for this service? etc.
As the old user experience adage goes: "spend $1 on usability and save $100* down-the-track".
Unfortunately, not so simple in my experience. That's a lot of time and money many companies can't or will not fork over. So, shortcuts and approximations are used as an alternative.