I think most people think that if the 2 Bloor-Danforth subway is extended from Kennedy Station northeast to Scarborough Town Centre and beyond, it would be with the same headways as the current Line 2.
Look at the 1 Yonge-University-Spadina. During the morning rush, trains get short-turned at St. Clair West Station. Plans have been made, unfortunately usually "postponed", to change the short-turn from St. Clair West to Glencairn Station. Still waiting to see that.
Anyways, if there isn't enough riders on such a Line 2 extension, they may have to put in a short-turn situation, as well. We may end up with a morning rush hour short-turn at Kennedy. Worse would be a short-turn during the evening rush hour. Even worse, a short-turn in the evening. Even more worse, a short-turn in all hours.
Consider this as well. The 4 Sheppard Line runs 4-car trains at 5 minute headways at all times, even during the rush hours. Both Line 1 & 2 run at 2 to 3 minute headways during the rush hours, except during the morning rush north of St. Clair West Station (at 5 minute headways), using 6-car trains.
The TTC is planning to do the same thing with the Spadina extension, with some trains short-turning at Downsview (Sheppard West).