I think this is indeed the main factor. Rather than any intention of Ecclestone to "kill" Group C was the decicive development that more and more works teams entered the stage. As usual and demonstrated frequently before and after in other series, with their potential they could speed up the screw of development so that cheap customer-friendly solutions were to fall behind quickly. In order to keep their customers in the game Porsche would have had to develop a new engine anyway and to me it is doubtful whether they would have delivered them a competitive solution even if the series had not switched to the 3.5 litre engines.
What Ecclestone certainly can be blamed for is, that obviously he was not unhappy with this development, otherweise he could have used his influence to push regulations that could have kept the privateers in the game (in particular something like C2 as mentioned above).
By 1990, there were some major hitters in Group C, with M-B, Porsche, Jaguar, Mazda, Toyota and Nissan spending their considerable budgets in sports car racing, rather than seeing the light and going F1 instead. We simply could not have that, could we? Jaguar (TWR) launched the turbocharged V6 out of the Metro 6R4. This engine had a very short life span, as it became obsolete when gr C went 3.5 atmo for 1991. As for Porsche, the 956/962 was still competitive, especially at Le Mans. Remember Pareja's Brun heartbreak, with retirement from 2nd place literally in the last 15 minutes of the Le Mans 24-hour race in 1990?
There were numerous privateer 962s around still, at the end of the fuel efficiency Gr C, with people like Richard Lloyd developing the 962 concept even further, including a carbon monocoque. For my money, the original Gr C could have gone on, with a healthy privateer base able to do well, even against the works efforts. The beauty of the original Gr C concept was that it provided so many ways to the Casbah, with big stock-block atmo engines taking on purpose built racing engines, atmospheric and super/turbocharged, with a wonderful diversity as the result.
Remember Procar? That was a 3.5 atmo manufacturer's class that backfired. And how about the power struggle around the Vingt-Quatre Heures, with the ACO standing up to the FIA and acquiring the land to build the Mulsanne chikanes, thus keeping their autonomy. Too bad sports car racing did not have the political clout to stand up to the FIA and its commercial rights holder.
Edited by doc knutsen, 17 October 2015 - 13:53.