“They” have not put money into transfers time and time again, though. The money was generated by the club, and the club could have spent even more (particularly during the late 2000s when we had the chance to build a long-lasting dynasty, and on infrastructure) if not for the Glazers' debt, dividend and strategic review payments.
Yes, they are to blame; the buck stops with them, ultimately. The fact that they couldn't find highly competend people to run the club, even after a decade, speaks to their failings as owners of the sporting institution. One might argue that Manchester United has been the most under-performing club of the modern era (relative to revenues), as a direct consequence of their ownership of the club. Matter of fact, I can't think of even one way they've benefited the club (in contrast with the Agnellis at Juventus, Berlusconi at Milan, Mansour at Manchester City, Abramovich at Chelsea et cetera). There's only a loooong list of cons with this lot.