You know the argument doesn't have merit when you're bringing a third player in
The simple fact is that your argument are based on what ifs and assumptions such as 'Yasir will probably fail in SA,' 'Ashwin has improved and will do better on second and third tries' etc etc
We have a sample set already and based on that Ashwin has been totally ineffective outside his comfort zone and at the best he can play a containing role which doesn't help a side like India on overseas tours. Basically don't bet money on Ashwin winning you any matches such as Yasir did in Oval and Lord. Yasir has a tendency to get smashed when the conditions don't favor and dominating batsmen are in the opposition, like many leg spinners before him.
Overall both have been disappointing abroad but only one has shown any capability of having a match winning streak.
In any case Ashwin had a terrible first tour of Australia. In his second tour of Australia he did better. Didn't take many wickets or set the world alight but bowled good lines and at times put pressure on the opposition even gf figures were still mediocre. Maybe on his third try he might finally crack it in Australia like Kumble did. Nothing to suggest Ashwin can dominate in England or South Africa