Things might have been different if Desilu hadn't had both shows (TOS and M:I) in concurrent production. If they'd had only one then it mightn't have been such a drain on resources. But they hit the jackpot (so to speak) in getting both series sold and only one would really last, possibly and partly because it had broader appeal and was more easily accessible for a general audience.
Desilu had three shows in production when the studio was bought by Paramount. Don't forget "Mannix."