My old s-100 ~1978-1985 could use the 50 (teletype) and "Kansas City Standard" at 300baud but was capable to 2400baud; so 1200 never was "the" baud rate of choice. Through the 1970's I worked on many ASCII terminals that used DEC, Wyse, and IBM protocols and could be set to communicate at least up to 9600 and some may have been capable up to ~32,000.
The 300, 1200, 2400, and eventually 9600baud were only limitations to the budget of home economics. From my own experience, much faster baud rates existed in the ($)commercial world at least 15-20 years earlier than 1980 - probably even sooner.
The telephone companies would support what ever modem rate you wanted - for a price. As I remember it, telephone companies charged between $1000-3000 per month (plus a usage fee) for a T1/T2 line in the 1980's.
Fast modems have been in existence since the stone age - just not affordable. In ~1991 the Govt hospital I worked for paid about $8000 (~$15k in today's money) for a T1 modem (available from ~1960). So, fast modems remained pricey and well above the average persons budget until the Internet.