Their goals were completely different,one wanted to be a pokemon master and the other wanted to be the strongest mega evolution trainer,there's no conflict between the two at all and neither grow or develop off each other,Alain just came off as more of a league rival but with more screentime.Them being involved with TF doesn't change anything.May and Drew actually had a developing rivalry,had conflict and shared the same goal.
Alain really didn't need to show up in the main plot he was fine in those XY specials.Adding him in the main plotline was pretty much fanservice,there was no real reason for him to enter into the Kalos league because his goal isn't to become an elite four member or pokemon champion,they just brought him in as a roadblock to keep Ash from winning the league.Ash's second best rival will always be Gary and that more than likely won't ever change.The two have history with each other when they were younger.
They both shared the same goal,even though they didn't battle much they still had good interactions whenever they met and there was conflict along with comedy which is better than anything I seen between Ash/Alain.Lastly what makes Alain or Sawyer better rivals than Barry,Stephan,Morrison,Harrison,Tyson and Trip?
The world rivals has lots of definitions "a person with the same goal" is just one of them. "A person that equals other in quality" "a competitor in the same competition" etc.
Alain develops, without this rivalry he would feel no remorse in MES4. And MOST of all, without him the TF plot would never exist to begin with.
Adding him the main plot was necessary to advance to the TF arc, he was always intended to be added to the main series eventually.
Gary's rivalry was pretty atrocious in gen 1, as they had practically no conclusion to their buildup, that happened in Johto and even then it wasn't developed as well imo. Nevertheless, it was amazing to see it finally conclude at that time, but it didn't age well.
"Barry,Stephan,Morrison,Harrison,Tyson and Trip"
Barry was just a secondary gag rival who had some development losing to Paul i guess.
Stephan was pretty ok.
Morrision, Harrision and Tyson had the same problem as they were introduced at the last minute and they didn't manage to invest me in their rivalry as much.
Trip was just discount Paul and it was pretty annoying to see Ash be an underdog again.
Knowing Alain's story and character had me invested in his relationship with Ash, as for Sawyer, his rivalry was really well developed in the context he was the "Kouhai" well if you understand that.