They thought the fad would last forever, and it didn't.
Agreed. They were rookies, and acted like it.
TSR's general policy of suing anything that moved certainly didn't help matters, in that doing so served to sour and-or drive away a significant chunk of their core (and thus most important) fan/customer base; even worse was that said core fans/customers were often the exact same people who were being C&Ded and-or sued.
I agree that it certainly didn't help matters, but what directly drove them under was owing tens of millions of dollars to Random House and other creditors because TSR was printing books they couldn't sell in order to secure advance money. As well, they had a terrible financing arrangement that locked up their money right at the start of the fiscal year, making them unable to cope with contingencies. From its beginning, TSR practiced the "kicking the can" method of financial management, which was inevitably going to bite it in the butt.
Very broadly put, the TSR-era goofs were mostly due to corporate blunders while the WotC-era goofs were (and still are) mostly due to corporate greed.
Blunders are to some extent forgivable. Greed isn't.
I don't see it this way at all. Corporate greed is a given for every successful company in a capitalist system, in the sense that maximizing profit/growth is a survival imperative. WotC, both before and under Hasbro, has also undertaken some unusually
non-greedy actions. For example, one of WotC's first actions after purchasing TSR was to settle its debts, even going beyond contractual obligations (e.g. returning original artwork to the artists for resale). Under Hasbro, the OGL and Roll20 may have been strategic (and thus arguably greedy) in inspiration, but certainly stand out as usual as far as large corporations go; you're not going to see Disney or Sony dump a huge amount of their core content into the Creative Commons any time soon.
I also think blunders are not necessarily less forgivable. A lot of people lost jobs and were taken advantage of through TSR's mismanagement, and after awhile the line between "blunder" and "greed" becomes pretty blurred. It wasn't a blunder that TSR tried to remove Arneson's name from AD&D, for example.