Joe - since you seem to know a lot about the history of the actual companies. Do you think TSR would have failed without Magic driving it in to the ground? Seems to me like the sudden appearance and popularity of Magic had a negative impact on other segments of the gaming hobby, and between that and TSR's attempt to enter the CCG market (Spell something?), that's what ultimately did TSR in.
Actually I am more a buff of corporate failures - not just those relating to the gaming industries (International Harvester being one of my favorites).
In the long run TSR probably would have survived. They suffered from a business management team who didn't understand the market or gaming (sound familiar?). Once they got to the point where they were at in the late 1990s - the management team would have sold their interest, and the company would have likely continued on. Cutting back and returning to the roots. Instead it was purchased by WotC who initially might have actually been good stewards...however shortly after WotC purchased TSR - they were bought by Hasbro (and the return of big management). Up until the sale, a lot of the TSR creative policies remained in place - even though they had decided to ax many of the settings (Dark Sun, Planescape and Spelljammer). Those cuts were not unexpected.
One thing which does not fully pass the smell test though is that when TSR was sold - they were actually working on a 3rd edition which incorporated many of the more popular Players Option rules. When 3rd was released by WotC - they touted it as having 3 years of development. D&D3 was so broke though, that they felt the need to release 3.5 a few years later to fix what they spent 3 years developing. Playtesting would have taken care of most the problems which were related to the 3 versus 3.5 versions...so the 3 year development cycle doesn't pass the sniff test. However a 1 year cycle would explain a lot of the issues (Hasbro not thinking D&D 3 was different enough and forcing changes). There is additional anecdotal evidence as well in regards to the development of Neverwinter Nights. The initial version was actually very close to release based on the working version of D&D 3 which was coopted through TSR's development team. Hasbro however forced the change from the original version to their version - this ended up causing a reworking of Neverwinter Nights game engine - this ended up delaying the release (and causing problems for the publisher due to loosing access to money which would have been generated by its release). For a good idea of what the 3rd edition was originally to be like - you can take a look at Baldur's Gate 2.
Partha's fall was largely tied to the loss of the D&D license for their miniatures. When that happened, it created somewhat of a perfect storm and they were purchased in much the same way as TSR (I still think that WotC wanted to strong arm Partha as well in order to purchase them - though I have no information regarding that). In return though, they were picked off by FASA since other than the D&D line...Battletech/Shadowrun were what kept Partha alive. Since FASA had the bargaining chips - they could pretty well dictate their own terms (and did).
FASA simply pulled the cord. They thought the RPG/Wargame market would be killed off by computers - so they cashed out and sold off what they could. The corporation still exists the last time I checked (couple years ago I guess) and still holder of several of their trademarks. Some are licensed off in a bit of creative accounting - others are collecting dust. At the time that they got out of things - Microsoft was paying them quite well as were a number of their other holdings. When they decided to shut things down, Microsoft bought their video game interests outright (and provided more than enough capital for Wizkids...which later lead to some additional creative accounting tricks).