well yeah sure cgi aint cheap but no doubt they would be paying much higher salaries than they are now if they had to negotiate contracts beyond series 7 where as cgi is basically a fixed cost its not like whoever does the cgi will want paying double for same amount of work in future seasons

saying that i could see them doing a spin off around roberts rebellion era will save a bunch of $ on cgi dragons on top of paying the new cast much lower wages. i mean i could be wrong and maybe there are other reasons they cutting the last 2 seasons short instead of making a few more full length series but thats my best guess