TA has its South-American hub in LIM, which is not served by any *A carrier except AC.
Without RG, most Brasilian cities are virtually inaccessible (except for a few served by TP, and only from LIS, hardly a convenient routing for many.
As pointed out by iwillflytheworld, many South-Am to South-Am connections on TA are difficult and take all day. Even North-South, connecting from NA to deep SA means a double change which also takes all day (i.e. leave the US very early morning to arrive middle of the night next day.
TA would indeed be a good addition to *A but in no way can they be seen as a suitable replacement for RG.
South-Am is primarily OW territory, followed, at some distance, by ST. *A offering is already not that great but would become extremely poor without RG. AC does cover a reasonable number of destinations, but going to YYZ to go to South Am is hardly what you would describe as convenient unless you are in Eastern Canada or North-East US.