TS will never opt for another satellite outside 83 degrees. And using a mono-block LNB will be a very difficult step as they will need to change LNB of millions of Dishes. Instead of this they can switch to HEVC and change STB in phase manner if they want to use a single satellite until they get a replacement for Insat 4A.
I feel Insat 4A will work for more years, and ISRO is also monitoring it, Insat 4A also has many C band channels so they will have to go for a new satellite in same orbit. So i feel ISRO will launch a new satellite this year GSAT 11 and GSAT 20.