It's not a theory since the official documents on it have been declassified nowadays, you can find the details on wikipedia, short version is they tried to get

It's not a theory since the official documents on it have been declassified nowadays, you can find the details on wikipedia, short version is they tried to get him killed but after a couple of years they realized he was not a very good military tactician yet still imposed his ideas/choices to the generals, so having him alive was beneficial to the allies. Unsurprisingly this is roughly at this point that German's generals plots to kill him from inside started, although their reasons were more complicated.